Improve the diagnostic when a new declaration conflicts with a using shadow

Improve the diagnostic when a new declaration conflicts with a using shadow
declaration.  Rename note_using_decl to note_using, which is possibly less confusing.
Add a test for non-class-scope using decl collisions and be sure to note the case
we can't diagnose yet.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@91057 91177308-0d34-0410-b5e6-96231b3b80d8