Move diagnostic enums into Basic.

Move enums from */*Diagnostic.h to Basic/Diagnostic*.h.  Basic/AllDiagnostics.h
needs all the enums and moving the sources to Basic prevents a Basic->*->Basic
dependency loop.  This also allows each Basic/Diagnostics*Kinds.td to have a
header at Basic/Diagnostic*.h (except for Common).  The old headers are kept in place since other packages are still using them.

llvm-svn: 348685