[CIR][NFC] Move LoweringPrepare into CIRGen

[lanza]

Move LP into CIRGen and give it a handle on the CIRGenModule. A lot of
code has been duplicated from CIRGen into cir/Dialect/Transforms in
order to let LP live there, but with more necessary CIRGen features
(e.g. EH scope and cleanups) going to be used in LP it doesn't make
sense to keep it separate. Add this patch that just refactors
LoweringPrepare into the CIRGen directory and give it a handle on the
CGM.

[github-actions]

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[bcardosolopes]

When you proposed this we discussed factoring out the CGM bits necessary and just passing it to LoweringPrepare, I'm curious what your findings were or at least some indication of what are the bits you really need there that justify this change, so far I have found none in the description of this PR.

CIRGen features (e.g. EH scope and cleanups)

This doesn't sound right, I've implemented most bits in CFGFlatten without needing CGM

[bcardosolopes]

Marking as blocked on the question in previous comment

[bcardosolopes]

I actually dig up a bit myself and this makes a lot of sense, I see now the point about code duplication - let's start with this and later we can factor out pieces as we go. Thanks for the refactoring.

[bcardosolopes]

Unfortunately I had to revert: I was taking one more deep review in tree and it's just super weird that none of the previous passes need CGM, including things that can done on top of CIR without AST support, and some later pass depend on CGM. My take is that the needed pieces in CGM need to be factored out and reused between LP and CGM, this is probably the most clean way to go about this, we can chat more about it early next week. Sorry for the churn.