[COFF] Don't produce weak aliases in import libraries

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.

Differential Revision: https://reviews.llvm.org/D36634

llvm-svn: 310992