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

[Nick Desaulniers 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);
+  if (Demangled)
+    Str = Demangled;
+  const DeclarationNameInfo target(&S.Context.Idents.get(Str), AL.getLoc());
+  LookupResult LR(S, target, Sema::LookupOrdinaryName);
+  if (S.LookupQualifiedName(LR, S.getCurLexicalContext()))
+    for (NamedDecl *ND : LR)
+      ND->markUsed(S.Context);
----------------
nickdesaulniers wrote:

I believe you can put `alias` attributes on non-functions: https://godbolt.org/z/s1PbojYr4.

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