Region inference: split results from RegionInferenceContext

[amandasystems]

What?

This PR turns RegionInferenceContext into a builder that produces the inferred region values, InferredRegions. The resulting struct implements the public API of RegionInferenceContext and replaces it in consumers.rs. RegionInferenceContext::solve() now consumes (moves) the inference context. It is completely private to region inference.

Why?

• RegionInferenceContext has become a huge dump for various values people want to access.
• region_infer itself is a very large file that's difficult to find your way around.
• Polonius wants to skip as much of region inference as possible, since most of it is redundant. This makes that possible
• No method now need warnings about being called after region inference or else they'll panic
• RegionInferenceContext now takes almost all of its fields by reference
• We can probably drop at least some stuff earlier
• Data dependencies during region inference are now made more explicit

Knock-on effects

• Region constraint (blame) search is now independent of a RegionInferenceContext
• Less data is retained after region inference. This may or may not lead to minor improvements in memory use.
• RegionInferenceContext now almost exclusively contains references to values, as opposed to owning them. This addresses most of fn compute_closure_requirements_modulo_opaques shouldn't clone all its inputs #146079
• This reduces the coupling between region inference and Polonius-next, making it possible to skip region inference if it is not needed to check universal regions or type tests.
• region_infer gains two child modules and becomes a lot less of a behemoth.

Detailed overview of changes

• new modules: region_infer::constraint_search and region_infer::universal_regions.
• handle_placeholders now consumes less input and does constraint rewriting via mutable references
• consumers.rs now has a InferenceResults instead of a RegionInferenceContext
• The public methods eval_{equal, outlives} are moved to InferredRegions
• Constraint (blame) search, MIR dumping, graphviz rendering, and checking universal regions now all get their own builder-style structs, and the methods for their respective tasks are moved onto them
• The option for region inference results in ConsumerOptions is now called InferredRegions
• calculate_borrows_out_of_scope_at_location of course now takes InferredRegions
• LivenessValues for free regions is now initialised as live at all points along with the constraint rewriting during placeholder handling, as opposed to during construction of RegionInferenceContext
• Some trace logging, especially during constraint search, is lost since the data required to print the logs was only necessary to print the logs and never used otherwise.

r? @lcnr

Closes #146079.