[clang] [Sema] Mark alias/ifunc targets used and consider mangled names (PR #87130)

[Aaron Ballman via cfe-commits]

================
@@ -1980,6 +1981,23 @@ static void handleWeakRefAttr(Sema &S, Decl *D, const ParsedAttr &AL) {
   D->addAttr(::new (S.Context) WeakRefAttr(S.Context, AL));
 }
 
+// Mark alias/ifunc target as used. For C++, we look up the demangled name
+// ignoring parameters. This should handle the majority of use cases while
+// leaveing false positives for namespace scope names and false negatives in
+// the presence of overloads.
+static void markUsedForAliasOrIfunc(Sema &S, Decl *D, const ParsedAttr &AL,
+                                    StringRef Str) {
+  char *Demangled = llvm::itaniumDemangle(Str, /*ParseParams=*/false);
----------------
AaronBallman wrote:

> I added a `S.getASTContext().getCXXABIKind() != TargetCXXABI::Microsoft` check.
> 
> Note: while `alias` works for Windows, it is only checked in tests CodeGenCXX/visibility-dllstorageclass.cpp/CodeGen/debug-info-alias-pointer.c . I don't consider an internal linkage target used or cared by users.

I'm not certain why there's so much resistance to doing this when three people have pointed out the concern. It still works for Windows. e.g.,
```
F:\source\llvm-project>cat "C:\Users\aballman\OneDrive - Intel Corporation\Desktop\test.cpp"
namespace NS {
  extern "C" int puts(const char *);
  void f() { puts("I did the thing!"); }
}

void func() __attribute__((alias("?f at NS@@YAXXZ")));

int main() {
  func();
}

F:\source\llvm-project>llvm\out\build\x64-Debug\bin\clang.exe "C:\Users\aballman\OneDrive - Intel Corporation\Desktop\test.cpp"

F:\source\llvm-project>a.exe
I did the thing!
```
We should support it properly rather than only halfway, *especially considering the context of the bug report*.

https://github.com/llvm/llvm-project/pull/87130