Rework how CIndex handles diagnostics. Rather than using a callback,

we attach diagnostics to translation units and code-completion
results, so they can be queried at any time.

To facilitate this, the new StoredDiagnostic class stores a diagnostic
in a serializable/deserializable form, and ASTUnit knows how to
capture diagnostics in this stored form. CIndex's CXDiagnostic is a
thin wrapper around StoredDiagnostic, providing a C interface to
stored or de-serialized diagnostics.

I've XFAIL'd one test case temporarily, because currently we end up
storing diagnostics in an ASTUnit that's never returned to the user
(because it contains errors). I'll introduce a temporary fix for this
soon; the real fix will be to allow us to return and query invalid ASTs.

llvm-svn: 96592