[analyzer] NFC: Move PathDiagnostic classes to libAnalysis.

At this point the PathDiagnostic, PathDiagnosticLocation, PathDiagnosticPiece
structures no longer rely on anything specific to Static Analyzer, so we can
move them out of it for everybody to use.

PathDiagnosticConsumers are still to be handed off.

Differential Revision: https://reviews.llvm.org/D67419

llvm-svn: 371661