Return a DIDescriptor from CGDebugInfo::getDeclarationOrDefinition...

... instead of a DIScope that might wrap something that's not actually
a DIScope (eg. a DIGlobalVariable);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@221481 91177308-0d34-0410-b5e6-96231b3b80d8