Alfredo Di Napoli authored Apr 06, 2021 and Marge Bot committed Apr 29, 2021

This commit adds GhcMessage and ancillary (PsMessage, TcRnMessage, ..)
types.
These types will be expanded to represent more errors generated
by different subsystems within GHC. Right now, they are underused,
but more will come in the glorious future.

See
https://gitlab.haskell.org/ghc/ghc/-/wikis/Errors-as-(structured)-values
for a design overview.

Along the way, lots of other things had to happen:

* Adds Semigroup and Monoid instance for Bag

* Fixes #19746 by parsing OPTIONS_GHC pragmas into Located Strings.
  See GHC.Parser.Header.toArgs (moved from GHC.Utils.Misc, where it
  didn't belong anyway).

* Addresses (but does not completely fix) #19709, now reporting
  desugarer warnings and errors appropriately for TH splices.
  Not done: reporting type-checker warnings for TH splices.

* Some small refactoring around Safe Haskell inference, in order
  to keep separate classes of messages separate.

* Some small refactoring around initDsTc, in order to keep separate
  classes of messages separate.

* Separate out the generation of messages (that is, the construction
  of the text block) from the wrapping of messages (that is, assigning
  a SrcSpan). This is more modular than the previous design, which
  mixed the two.

Close #19746.

This was a collaborative effort by Alfredo di Napoli and
Richard Eisenberg, with a key assist on #19746 by Iavor
Diatchki.

Metric Increase:
    MultiLayerModules