test/CodeGenCXX/dllexport.cpp: we already correctly emit b() even when
it's not used, because CodeGenModule::EmitGlobal consults
ASTContext::DeclMustBeEmitted via CodeGenModule::MayDeferGeneration.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208936 91177308-0d34-0410-b5e6-96231b3b80d8