The constructor for ASTUnit now takes a DiagnosticClient*, allowing uses of ASTUnit to specify

alternate DiagnosticClients. To match this API, ASTUnit::LoadFromPCHFile() now takes a corresponding
DiagnosticClient* argument as well. The DiagnosticClient object is destroyed when the ASTUnit object
is destroyed.

The CIndex library now uses this API to create a 'IgnoreDiagnosticsClient' that simply silences
diagnostics when using the clang_createTranslationUnitFromSourceFile() function. This fixes
<rdar://problem/7312058>. This API can change in the future as we add more flexibility for clients.

llvm-svn: 84539