Rename getMessages and getErrorMessages to avoid import collisions

See also #19905 .

It has been brought to my attention that is slightly annoying that we have a name clash in a handful of function names:

• getMessages
• getErrorMessages
• getWarningMessages

We use these both in the parser code and in the GHC.Types.Error code, for the Messages newtype and its getters.

As it's fairly obvious, there are 2 ways to attack this: we can either rename the functions in GHC.Types.Error or the ones in GHC.Parser.y & co. I have tried briefly to resolve the clash by renaming the latter, but that proved to be more invasive than I anticipated and it would require an haddock bump (le sigh).

I will try next to rename the former, and see where it brings us.