Add compatibility for clang 3.5 (svn)

- FunctionDecl::getResultType() was renamed to FunctionDecl::getCallResultType()
- We were misusing DiagnosticsEngine::getCustomDiagID as it should always
  take a constant string.  Switch to using formatted output to make this work
  on both clang 3.5 and previous versions.
- Correct a copy-paste error where we were printing the name of the output
  directory instead of the name of the temporary directory in an error
  message.