[LLD] [COFF] Fix mingw comdat associativity for leader symbols with a different name

For a weak symbol func in a comdat, the actual leader symbol ends up
named like .weak.func.default*. Likewise, for stdcall on i386, the symbol
may be named _func@4, while the section suffix only is "func", which the
previous implementation didn't handle.

This fixes unwinding through weak functions when using
-ffunction-sections in mingw environments.

Differential Revision: https://reviews.llvm.org/D84607

(cherry picked from commit 343ffa70)