This commit introduces in GHC.Types.Error.Codes the function

  constructorCodes :: forall diag. (...) => Map DiagnosticCode String

which computes a collection of all the diagnostic codes that correspond
to a particular type. In particular, we can compute the collection of
all diagnostic codes emitted by GHC using the invocation

  constructorCodes @GhcMessage

We then make use of this functionality in the new "codes" test which
checks consistency and coverage of GHC diagnostic codes.
It performs three checks:

  - check 1: all non-outdated GhcDiagnosticCode equations
    are statically used.
  - check 2: all outdated GhcDiagnosticCode equations
    are statically unused.
  - check 3: all statically used diagnostic codes are covered by
    the testsuite (modulo accepted exceptions).