[COFF] Cope with weak aliases produced by GNU tools

When GNU tools create a weak alias, they produce a strong symbol
named .weak.<weaksymbol>.<relatedstrongsymbol>.

GNU ld allows many such weak alternatives for the same weak symbol, and
the linker picks the first one encountered.

This can't be reproduced by assembling from .s files, since llvm-mc
produces symbols named .weak.<weaksymbol>.default in these cases.

Differential Revision: https://reviews.llvm.org/D52601

llvm-svn: 343704