[PATCH] MC: Support COFF image-relative MCSymbolRefs

[Michael Spencer]

  Is there any precedent for foo at imgrel? Also, I'm not code owner for Windows codegen, @asl is.

  If imgrel is new, it needs to be documented as an extension to the as syntax. Other than that it LGTM.

http://llvm-reviews.chandlerc.com/D635