Split diagnostics out into separate files.

Thanks to the way we've set up our diagnostics engine, there's not actually
a reason for /everything/ to get rebuilt when /one/ diagnostic changes.
I've split them up into five categories for now: Parse, Sema, SIL, IRGen,
and Frontend, plus a set of "Common" diagnostics that are used in multiple
areas of the compiler. We can massage this later.

No functionality change, but should speed up compile times!

Swift SVN r12438

lib/Frontend/CompilerInvocation.cpp

@@ -12,6 +12,7 @@
 
 #include "swift/Frontend/Frontend.h"
 
+#include "swift/AST/DiagnosticsFrontend.h"
 #include "swift/Subsystems.h"
 #include "swift/Driver/Options.h"
 #include "llvm/ADT/STLExtras.h"