[patch][pr17530] Prevent alias from pointing to weak aliases

[Rafael Espíndola]

The llvm side of this was LGTMed, but unfortunately I failed to
noticed that clang could produce aliases to weak aliases, so the patch
got reverted pending changes to clang.

With llvm rejecting aliases to weak aliases, we have to decide what to
do with it in clang.

One option is to error. That is fairly reasonable since the user can
easily fix the code (see r204823
in compiler-rt for example). The issue is gcc incompatibility.

What this patch does instead is skip past the weak alias in clang and
warn the user that it is probably not the expected semantics.

The llvm patch is attached for reference.

Cheers,
Rafael
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