ms inline asm: Don't add x86 segment registers to the clobber list.

Clang tries to check the clobber list but doesn't list segment registers in its
x86 register list. This fixes PR20343.
MS compatibility: always emit dllexported in-class initialized static data members (PR20140)

This makes us emit dllexported in-class initialized static data members (which
are treated as definitions in MSVC), even when they're not referenced.

It also makes their special linkage reflected in the GVA linkage instead of
getting massaged in CodeGen.

Differential Revision: http://reviews.llvm.org/D4563