[clang][driver][clang-cl] Support --precompile and -fmodule-* options in Clang-CL (#98761)

This PR is the first step in improving the situation for `clang-cl`
detailed in [this LLVM Discourse
thread](https://discourse.llvm.org/t/clang-cl-exe-support-for-c-modules/72257/28).
There has been some work done in #89772. I believe this is somewhat
orthogonal.

This is a work-in-progress; the functionality has only been tested with
the [basic 'Hello World'
example](https://clang.llvm.org/docs/StandardCPlusPlusModules.html#quick-start),
and proper test cases need to be written. I'd like some thoughts on
this, thanks!

Partially resolves #64118.

clang/docs/StandardCPlusPlusModules.rst

@@ -398,6 +398,16 @@ BMIs cannot be shipped in an archive to create a module library. Instead, the
 BMIs(``*.pcm``) are compiled into object files(``*.o``) and those object files
 are added to the archive instead.
 
+clang-cl
+~~~~~~~~
+
+``clang-cl`` supports the same options as ``clang++`` for modules as detailed above;
+there is no need to prefix these options with ``/clang:``. Note that ``cl.exe``
+`options to emit/consume IFC files <https://devblogs.microsoft.com/cppblog/using-cpp-modules-in-msvc-from-the-command-line-part-1/>` are *not* supported.
+The resultant precompiled modules are also not compatible for use with ``cl.exe``.
+
+We recommend that build system authors use the above-mentioned ``clang++`` options  with ``clang-cl`` to build modules.
+
 Consistency Requirements
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
@@ -1387,13 +1397,6 @@ have ``.cppm`` (or ``.ccm``, ``.cxxm``, ``.c++m``) as the file extension.
 However, the behavior is inconsistent with other compilers. This is tracked by
 `#57416 <https://github.com/llvm/llvm-project/issues/57416>`_.
 
-clang-cl is not compatible with standard C++ modules
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
-``/clang:-fmodule-file`` and ``/clang:-fprebuilt-module-path`` cannot be used
-to specify the BMI with ``clang-cl.exe``. This is tracked by
-`#64118 <https://github.com/llvm/llvm-project/issues/64118>`_.
-
 Incorrect ODR violation diagnostics
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

clang/docs/UsersManual.rst

@@ -4745,6 +4745,12 @@ Execute ``clang-cl /?`` to see a list of supported options:
       -flto=<value>           Set LTO mode to either 'full' or 'thin'
       -flto                   Enable LTO in 'full' mode
       -fmerge-all-constants   Allow merging of constants
+      -fmodule-file=<module_name>=<module-file>
+                              Use the specified module file that provides the module <module_name>
+      -fmodule-header=<header>
+                              Build <header> as a C++20 header unit
+      -fmodule-output=<path>
+                              Save intermediate module file results when compiling a standard C++ module unit.
       -fms-compatibility-version=<value>
                               Dot-separated value representing the Microsoft compiler version
                               number to report in _MSC_VER (0 = don't define it; default is same value as installed cl.exe, or 1933)

clang/include/clang/Driver/Options.td

@@ -3105,7 +3105,7 @@ def fmodules_user_build_path : Separate<["-"], "fmodules-user-build-path">, Grou
   HelpText<"Specify the module user build path">,
   MarshallingInfoString<HeaderSearchOpts<"ModuleUserBuildPath">>;
 def fprebuilt_module_path : Joined<["-"], "fprebuilt-module-path=">, Group<i_Group>,
-  Flags<[]>, Visibility<[ClangOption, CC1Option]>,
+  Flags<[]>, Visibility<[ClangOption, CLOption, CC1Option]>,
   MetaVarName<"<directory>">,
   HelpText<"Specify the prebuilt module path">;
 defm prebuilt_implicit_modules : BoolFOption<"prebuilt-implicit-modules",
@@ -3114,11 +3114,11 @@ defm prebuilt_implicit_modules : BoolFOption<"prebuilt-implicit-modules",
   NegFlag<SetFalse>, BothFlags<[], [ClangOption, CC1Option]>>;
 
 def fmodule_output_EQ : Joined<["-"], "fmodule-output=">,
-  Flags<[NoXarchOption]>, Visibility<[ClangOption, CC1Option]>,
+  Flags<[NoXarchOption]>, Visibility<[ClangOption, CLOption, CC1Option]>,
   MarshallingInfoString<FrontendOpts<"ModuleOutputPath">>,
   HelpText<"Save intermediate module file results when compiling a standard C++ module unit.">;
 def fmodule_output : Flag<["-"], "fmodule-output">, Flags<[NoXarchOption]>,
-  Visibility<[ClangOption, CC1Option]>,
+  Visibility<[ClangOption, CLOption, CC1Option]>,
   HelpText<"Save intermediate module file results when compiling a standard C++ module unit.">;
 
 defm skip_odr_check_in_gmf : BoolOption<"f", "skip-odr-check-in-gmf",
@@ -3302,8 +3302,10 @@ def fretain_comments_from_system_headers : Flag<["-"], "fretain-comments-from-sy
   Visibility<[ClangOption, CC1Option]>,
   MarshallingInfoFlag<LangOpts<"RetainCommentsFromSystemHeaders">>;
 def fmodule_header : Flag <["-"], "fmodule-header">, Group<f_Group>,
+  Visibility<[ClangOption, CLOption]>,
   HelpText<"Build a C++20 Header Unit from a header">;
 def fmodule_header_EQ : Joined<["-"], "fmodule-header=">, Group<f_Group>,
+  Visibility<[ClangOption, CLOption]>,
   MetaVarName<"<kind>">,
   HelpText<"Build a C++20 Header Unit from a header that should be found in the user (fmodule-header=user) or system (fmodule-header=system) search path.">;
 
@@ -5955,6 +5957,7 @@ def _output : Separate<["--"], "output">, Alias<o>;
 def _param : Separate<["--"], "param">, Group<CompileOnly_Group>;
 def _param_EQ : Joined<["--"], "param=">, Alias<_param>;
 def _precompile : Flag<["--"], "precompile">, Flags<[NoXarchOption]>,
+  Visibility<[ClangOption, CLOption]>,
   Group<Action_Group>, HelpText<"Only precompile the input">;
 def _prefix_EQ : Joined<["--"], "prefix=">, Alias<B>;
 def _prefix : Separate<["--"], "prefix">, Alias<B>;

clang/test/Driver/cl-cxx20-modules.cppm

@@ -0,0 +1,8 @@
+// RUN: %clang_cl /std:c++20 --precompile -### -- %s 2>&1 | FileCheck --check-prefix=PRECOMPILE %s
+// PRECOMPILE: -emit-module-interface
+
+// RUN: %clang_cl /std:c++20 --fmodule-file=Foo=Foo.pcm -### -- %s 2>&1 | FileCheck --check-prefix=FMODULEFILE %s
+// FMODULEFILE: -fmodule-file=Foo=Foo.pcm
+
+// RUN: %clang_cl /std:c++20 --fprebuilt-module-path=. -### -- %s 2>&1 | FileCheck --check-prefix=FPREBUILT %s
+// FPREBUILT: -fprebuilt-module-path=.