[Sema] Mark alias/ifunc targets used and consider mangled names

https://reviews.llvm.org/D54188 marked "alias" targets as used in C to
fix -Wunused false positives. This patch extends the approach to handle
mangled names to support global scope names in C++ and the
`overloadable` attribute in C.

In addition, we mark ifunc targets as used to fix #63957.

While our approach has false negatives for namespace scope names, the
majority of alias/ifunc C++ uses (global scope with no overloads) are
handled.

Note: The following function with internal linkage but C language
linkage type is mangled in Clang but not in GCC. This inconsistency
makes alias/ifunc difficult to use in C++ with portability (#88593).
```
extern "C" {
static void f0() {}
// GCC: void g0() __attribute__((alias("_ZL2f0v")));
// Clang: void g0() __attribute__((alias("f0")));
}
```

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

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