[c++2a] Improve diagnostic for use of declaration from another TU's
global module fragment.

We know that the declaration in question should have been introduced by
a '#include', so try to figure out which one and suggest it. Don't
suggest importing the global module fragment itself!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@358631 91177308-0d34-0410-b5e6-96231b3b80d8