Fix a crasher in Template Diffing.

When support was added for declaration arguments, the case of variadic
declaration arguments was not supported.  This patch fixes that problem by
not crashing when certain ValueDecl's are null.

Patch by Olivier Goffart!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178610 91177308-0d34-0410-b5e6-96231b3b80d8