[COFF] Make the weak aliases optional

When creating an import library from lld, the cases with
Name != ExtName shouldn't end up as a weak alias, but as a real
export of the new name, which is what actually is exported from
the DLL.

This restores the behaviour of renamed exports to what it was in
4.0.

The other half of this commit, including test, goes into lld.

Differential Revision: https://reviews.llvm.org/D36633

llvm-svn: 310991