WinCOFF: Transform IR expressions featuring __ImageBase into image relative relocations

MSVC on x64 requires that we create image relative symbol
references to refer to RTTI data. Seeing as how there is no way to
explicitly make reference to a given relocation type in LLVM IR, pattern
match expressions of the form &foo - &__ImageBase.

Differential Revision: http://llvm-reviews.chandlerc.com/D2523