diagtool: simplify TreeView diagnostic classification

This utility doesn't need to know about the specifics of diagnostic levels.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211323 91177308-0d34-0410-b5e6-96231b3b80d8
Objective-C qoi. When Objective-C pointer mismatches with
a qualified-id type because pointer is object of a forward
class declaration, include this info in a diagnostic note.
// rdar://10751015


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211324 91177308-0d34-0410-b5e6-96231b3b80d8