Improve error recovery in the typechecker

Issue #16418 showed that we were carrying on too eagerly after a bogus
type signature was identified (a bad telescope in fact), leading to a
subsequent crash.

This led me in to a maze of twisty little passages in the typechecker's
error recovery, and I ended up doing some refactoring in TcRnMonad.
Some specfifics

* TcRnMonad.try_m is now called attemptM.

* I switched the order of the result pair in tryTc,
  to make it consistent with other similar functions.

* The actual exception used in the Tc monad is irrelevant so,
  to avoid polluting type signatures, I made tcTryM, a simple
  wrapper around tryM, and used it.

The more important changes are in

* TcSimplify.captureTopConstraints, where we should have been calling
  simplifyTop rather than reportUnsolved, so that levity defaulting
  takes place properly.

* TcUnify.emitResidualTvConstraint, where we need to set the correct
  status for a new implication constraint.  (Previously we ended up
  with an Insoluble constraint wrapped in an Unsolved implication,
  which meant that insolubleWC gave the wrong answer.

0 files changed, 0 insertions, 0 deletions