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

[Nick Desaulniers via cfe-commits]

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@@ -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);
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nickdesaulniers wrote:

> While we can add S.getASTContext().getCXXABIKind() != TargetCXXABI::Microsoft check for downstream users, it doesn't appear to add any value?

If we know the target ABI uses Microsoft mangling, then we should be using `microsoftDemangle` rather than `itaniumDemangle`.  There's also `llvm::demangle` which can try to figure this out.

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