[MC, COFF] Permit a variable to be redefined

Our assertions in WinCOFFStreamer had unexpected side effects resulting
in symbols getting unexpectedly marked as used.

This fixes PR28462.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@274941 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed