[ELF] For relative paths in INPUT() and GROUP(), search the directory of the current linker script before searching other paths

For a relative path in INPUT() or GROUP(), this patch changes the search order by adding the directory of the current linker script.
The new search order (consistent with GNU ld >= 2.35 regarding the new test `test/ELF/input-relative.s`):

1. the directory of the current linker script (GNU ld from Binutils 2.35 onwards; https://sourceware.org/bugzilla/show_bug.cgi?id=25806)
2. the current working directory
3. library paths (-L)

This behavior makes it convenient to replace a .so or .a with a linker script with additional input. For example, glibc

```
% cat /usr/lib/x86_64-linux-gnu/libm.a
/* GNU ld script
*/
OUTPUT_FORMAT(elf64-x86-64)
GROUP ( /usr/lib/x86_64-linux-gnu/libm-2.29.a /usr/lib/x86_64-linux-gnu/libmvec.a )
```

could be simplified as `GROUP(libm-2.29.a libmvec.a)`.

Another example is to make libc++.a a linker script:
```
INPUT(libc++.a.1 libc++abi.a)
```

Note, -l is not affected.

Reviewed By: psmith

Differential Revision: https://reviews.llvm.org/D77779

lld/ELF/ScriptLexer.h

@@ -40,13 +40,14 @@ public:
   bool inExpr = false;
   size_t pos = 0;
 
+protected:
+  MemoryBufferRef getCurrentMB();
+
 private:
   void maybeSplitExpr();
   StringRef getLine();
   size_t getLineNumber();
   size_t getColumnNumber();
-
-  MemoryBufferRef getCurrentMB();
 };
 
 } // namespace elf

lld/ELF/ScriptParser.cpp

@@ -290,22 +290,40 @@ void ScriptParser::addFile(StringRef s) {
   }
 
   if (s.startswith("/")) {
+    // Case 1: s is an absolute path. Just open it.
     driver->addFile(s, /*withLOption=*/false);
   } else if (s.startswith("=")) {
+    // Case 2: relative to the sysroot.
     if (config->sysroot.empty())
       driver->addFile(s.substr(1), /*withLOption=*/false);
     else
       driver->addFile(saver.save(config->sysroot + "/" + s.substr(1)),
                       /*withLOption=*/false);
   } else if (s.startswith("-l")) {
+    // Case 3: search in the list of library paths.
     driver->addLibrary(s.substr(2));
-  } else if (sys::fs::exists(s)) {
-    driver->addFile(s, /*withLOption=*/false);
   } else {
-    if (Optional<std::string> path = findFromSearchPaths(s))
-      driver->addFile(saver.save(*path), /*withLOption=*/true);
-    else
-      setError("unable to find " + s);
+    // Case 4: s is a relative path. Search in the directory of the script file.
+    std::string filename = std::string(getCurrentMB().getBufferIdentifier());
+    StringRef directory = sys::path::parent_path(filename);
+    if (!directory.empty()) {
+      SmallString<0> path(directory);
+      sys::path::append(path, s);
+      if (sys::fs::exists(path)) {
+        driver->addFile(path, /*withLOption=*/false);
+        return;
+      }
+    }
+    // Then search in the current working directory.
+    if (sys::fs::exists(s)) {
+      driver->addFile(s, /*withLOption=*/false);
+    } else {
+      // Finally, search in the list of library paths.
+      if (Optional<std::string> path = findFromSearchPaths(s))
+        driver->addFile(saver.save(*path), /*withLOption=*/true);
+      else
+        setError("unable to find " + s);
+    }
   }
 }
 

lld/test/ELF/linkerscript/input-relative.s

@@ -0,0 +1,44 @@
+# REQUIRES: x86
+## For a relative pathname in INPUT() or GROUP(), the parent directory of
+## the current linker script has priority over current working directory and -L.
+
+# RUN: rm -rf %t.dir && mkdir %t.dir && cd %t.dir
+
+# RUN: mkdir dir
+# RUN: llvm-mc -filetype=obj -triple=x86_64 %s -o a.o
+# RUN: echo '.globl b, cwd; b: cwd:' | llvm-mc -filetype=obj -triple=x86_64 - -o b.o
+# RUN: echo '.globl b, dir; b: dir:' | llvm-mc -filetype=obj -triple=x86_64 - -o dir/b.o
+# RUN: llvm-ar rc libb.a b.o
+# RUN: llvm-ar rc dir/libb.a dir/b.o
+
+## A relative pathname is relative to the parent directory of the current linker script.
+## The directory has priority over current working directory and -L.
+# RUN: echo 'INPUT(libb.a)' > dir/relative.lds
+# RUN: ld.lld -L. a.o dir/relative.lds -o - | llvm-nm - | FileCheck --check-prefix=DIR %s
+## GROUP() uses the same search order.
+# RUN: echo 'GROUP(libb.a)' > dir/relative1.lds
+# RUN: ld.lld -L. a.o dir/relative1.lds -o - | llvm-nm - | FileCheck --check-prefix=DIR %s
+
+# DIR: T dir
+
+## -l does not use the special rule.
+# RUN: echo 'INPUT(-lb)' > dir/cwd.lds
+# RUN: ld.lld -L. a.o dir/cwd.lds -o - | llvm-nm - | FileCheck --check-prefix=CWD %s
+# RUN: echo 'GROUP(-lb)' > dir/cwd1.lds
+# RUN: ld.lld -L. a.o dir/cwd1.lds -o - | llvm-nm - | FileCheck --check-prefix=CWD %s
+
+# CWD: T cwd
+
+## The rules does not apply to an absolute path.
+# RUN: echo 'INPUT(/libb.a)' > dir/absolute.lds
+# RUN: not ld.lld a.o dir/absolute.lds -o /dev/null
+
+## If the parent directory of the current linker script does not contain the file,
+## fall back to the current working directory.
+# RUN: cp libb.a libc.a
+# RUN: echo 'INPUT(libc.a)' > dir/fallback.lds
+# RUN: ld.lld a.o dir/fallback.lds -o /dev/null
+
+.globl _start
+_start:
+  call b