[modules] Factor .td-generated diagnostics stuff out into its own module; this
avoids users of AllDiagnostics.h from needing to pregenerate *all* generated
headers. Hopefully this will make my modules buildbot happier...


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215617 91177308-0d34-0410-b5e6-96231b3b80d8