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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213304 91177308-0d34-0410-b5e6-96231b3b80d8