DebugInfo: Don't emit a 'global variable' when a static member declaration is referenced without a definition, just ensure the enclosing class (with the static member declaration) is emitted.

Addresses PR21511 by emitting appropriate metadata rather than
faux-global definitions for a variable that doesn't have a definition.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222377 91177308-0d34-0410-b5e6-96231b3b80d8