[WebAssembly] WebAssembly doesn't support "protected" visibility

Implement the `hasProtectedVisibility()` hook to indicate that, like
Darwin, WebAssembly doesn't support "protected" visibility.

On ELF, "protected" visibility is intended to be an optimization, however
in practice it often [isn't], and ELF documentation generally ranges from
[not mentioning it at all] to [strongly discouraging its use].

[isn't]: https://www.airs.com/blog/archives/307
[not mentioning it at all]: https://gcc.gnu.org/wiki/Visibility
[strongly discouraging its use]: https://www.akkadia.org/drepper/dsohowto.pdf

While here, also mention the new Reactor support in the release notes.

clang/lib/Basic/Targets/WebAssembly.h

@@ -132,7 +132,14 @@ private:
   }
 
   bool hasExtIntType() const override { return true; }
+
+  bool hasProtectedVisibility() const override {
+    // TODO: For now, continue to advertise "protected" support for
+    // Emscripten targets.
+    return getTriple().isOSEmscripten();
+  }
 };
+
 class LLVM_LIBRARY_VISIBILITY WebAssembly32TargetInfo
     : public WebAssemblyTargetInfo {
 public:

llvm/docs/ReleaseNotes.rst

@@ -144,8 +144,14 @@ Changes to the AVR Target
 Changes to the WebAssembly Target
 ---------------------------------
 
-During this release ...
+* Programs which don't have a "main" function, called "reactors" are now
+  properly supported, with a new `-mexec-model=reactor` flag. Programs which
+  previously used `-Wl,--no-entry` to avoid having a main function should
+  switch to this new flag, so that static initialization is properly
+  performed.
 
+* `__attribute__((visibility("protected")))` now evokes a warning, as
+  WebAssembly does not support "protected" visibility.
 
 Changes to the OCaml bindings
 -----------------------------