COFF: Emit all MCSymbols rather than filtering out some of them

In particular, this means we emit non-external symbols defined to
variables, such as aliases or absolute addresses.

This is needed to implement /safeseh, and it appears there was some
confusion about what symbols to emit previously.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@190888 91177308-0d34-0410-b5e6-96231b3b80d8