d076608d58d1ec55016eb747a995511e3a3f72aa moved some deps around to avoid
cycles and left clang/Frontend/FrontendDiagnostic.h as a shim that
simply includes clang/Basic/DiagnosticFrontend.h. This PR inlines it so
that nothing in tree still includes clang/Frontend/FrontendDiagnostic.h.
Doing this will help prevent future layering issues. See #162865.
Frontend already depends on Basic, so no new deps need to be added
anywhere except for places that do strict dep checking.
3d3307ecd8