[ms-cxxabi] Thread GlobalDecls through to CodeGenModule::getFunctionLinkage.

This is so that we can give destructor variants different linkage later.

Differential Revision: http://llvm-reviews.chandlerc.com/D819

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183324 91177308-0d34-0410-b5e6-96231b3b80d8