[COFF] Adjust how we flag weak externals

This fixes PR36096.

Originally based on a patch by Martell Malone.

Differential Revision: https://reviews.llvm.org/D44357

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@337613 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed