Refactoring of Diagnostic class.

-Move the stuff of Diagnostic related to creating/querying diagnostic IDs into a new DiagnosticIDs class.
-DiagnosticIDs can be shared among multiple Diagnostics for multiple translation units.
-The rest of the state in Diagnostic object is considered related and tied to one translation unit.
-Have Diagnostic point to the SourceManager that is related with. Diagnostic can now accept just a
   SourceLocation instead of a FullSourceLoc.
-Reflect the changes to various interfaces.

llvm-svn: 119730