Fix PR9390 in not one, but two ways:

  1) When we do an instantiation of the injected-class-name type,
  provide a proper source location. This is just plain good hygiene.

  2) When we're building a NestedNameSpecifierLoc from a CXXScopeSpec,
  only return an empty NestedNameSpecifierLoc if there's no
  representation.

Both problems contributed to the horrible test case in PR9390 that I
couldn't reduce down to something palatable.

llvm-svn: 126961