Clang: Speed up requesting diagnostics

Register the translation unit and request diagnostics from the
clangbackend as soon as the project part is determined. There is no
reason to wait until the parser is finished for the highlighter.

Change-Id: Iebccbf59ebd205389462dcee97363746fb651bb2
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>

src/plugins/cpptools/cppprojects.h

@@ -247,4 +247,6 @@ private:
 
 } // namespace CppTools
 
+Q_DECLARE_METATYPE(CppTools::ProjectPart::Ptr)
+
 #endif // CPPPROJECTPART_H