[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. (Note: we should skip `UsingShadowDecl`, which would trigger an assertion failure in `ItaniumMangleContextImpl::mangleCXXName`. See regression test added by commit 1c2afbae9af22b58190c10e3517242d01d89d612.) In addition, we mark ifunc targets as used to fix #63957 (temporarily used by xz; ifunc was removed by https://github.com/tukaani-project/xz/commit/689ae2427342a2ea1206eb5ca08301baf410e7e0) 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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