Do not create a replaceable Variables MDNode for function forward decls.

These fields would need to be explicitly deleted before we RAUW the temporary
node anyway (this was done in cfe commit r222373). Instead, do not create
these useless nodes in the first place.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222434 91177308-0d34-0410-b5e6-96231b3b80d8