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

[Fangrui Song 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);
----------------
MaskRay wrote:

> alias(mangled) referring to non-existent definitions are rejected by another diagnostic.

It's an error
```
% fclang -c b.cc
b.cc:3:28: error: ifunc must point to a defined function
    3 | int i1(int) __attribute__((ifunc("f")));
      |                            ^
b.cc:3:28: note: the function specified in an ifunc must refer to its mangled name
b.cc:3:28: note: function by that name is mangled as "_Z1fv"
    3 | int i1(int) __attribute__((ifunc("f")));
      |                            ^~~~~~~~~~
      |                            ifunc("_Z1fv")
1 error generated.
```

The reported issue has nothing to do with security as all. It's about a `-Wunused-function` false positive. (https://git.tukaani.org/?p=xz.git;a=commitdiff;h=a37a2763383e6c204fe878e1416dd35e7711d3a9)

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