[COFF] Don't try to emit weak aliases on COFF

This comes up when a derived class destructor is equivalent to a base
class destructor defined in the same TU, and we try to alias them.

A COFF weak alias cannot satisfy a normal undefined symbol reference
from another TU. The other TU must also mark the referenced symbol as
weak, and we can't rely on that.

Clang already has a special case here for dllexport, but we failed to
realize that the problem also applies to other non-discardable symbols
such as those from explicit template instantiations.

Fixes PR25477.

llvm-svn: 252659

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