Refactor NLL constraint generation and most of polonius fact generation

[lqd]

As discussed in #118175, NLL "constraint generation" is only about liveness, but currently also contains legacy polonius fact generation. The latter is quite messy, and this PR cleans this up to prepare for its future removal:

• splits polonius fact generation out of NLL constraint generation
• merges NLL constraint generation to its more natural place, liveness
• extracts all of the polonius fact generation from NLLs apart from MIR typeck (as fact generation is somewhat in a single place there already, but should be cleaned up) into its own explicit module, with a single entry point instead of many.

There should be no behavior changes, and tests seem to behave the same as master: without polonius, with legacy polonius, with the in-tree polonius.

I've split everything into smaller logical commits for easier review, as it required quite a bit of code to be split and moved around, but it should all be trivial changes.

r? @matthewjasper

[jackh726]

Fairly straightforward refactor. r=me if you'd like

[lqd]

Thanks for taking a look, Jack! It's not particularly urgent as you'd expect, so it's completely fine to wait for a bit in case Matthew doesn't have time to look at this (or they can r=you).

[matthewjasper]

@bors r+

[bors]

📌 Commit f969af2 has been approved by matthewjasper

It is now in the queue for this repository.

[bors]

⌛ Testing commit f969af2 with merge caf7300...

[bors]

☀️ Test successful - checks-actions
Approved by: matthewjasper
Pushing caf7300 to master...

[rust-timer]

Finished benchmarking commit (caf7300): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.3%	[-2.5%, -2.1%]	3
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 672.931s -> 673.917s (0.15%)
Artifact size: 313.36 MiB -> 313.41 MiB (0.02%)