DebugInfo: Make CGDebugInfo::getStaticDataMemberDeclaration's argument type match the semantics

Rather than having a cast immediately inside the function, push that
type requirement out to the callers.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@188492 91177308-0d34-0410-b5e6-96231b3b80d8