When we decide to output a deferred decl, remember the llvm GlobalValue.

We can reuse it to avoid a DenseMap+StringMap lookup to find if it was already
emitted or not.

This fixes a 2010 TODO.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@196785 91177308-0d34-0410-b5e6-96231b3b80d8