[mc-coff] Resolve aliases when emitting COFF relocations

This is consistent with the ELF object writer.

Add some COFF tests that relocate against an alias.

Reviewers: espindola

Differential Revision: http://llvm-reviews.chandlerc.com/D1079

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186341 91177308-0d34-0410-b5e6-96231b3b80d8