Fix a crasher in Template Diffing.

Value depenedent expressions for default arguments cannot be evaluated.
Instead, use the desugared template type to get an argument expression that
can be used.  This is needed for both integer and declaration arguements.

Also, move this common code into a separate function.

Patch by Olivier Goffart!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178609 91177308-0d34-0410-b5e6-96231b3b80d8