Fix <rdar://problem/5969777> clang ObjC rewriter: #imported file name mysteriously commented out

This fix involved tightening up needToScanForQualifiers(), which predated ObjCQualifiedIdType.
It also includes a minor tweak to the code that searches for the beginning of the qualified type. If the AST contained the correct beginning of the declaration (i.e. the beginning of the declaration specifiers), this code would be more robust. Since it doesn't, we apply an adhoc heuristic. Note that DeclSpec contains this info...we just don't propagate it to the AST (at present).

llvm-svn: 51812