fix: unify macro call type with actual type in comptime blocks

[asterite]

Description

Problem

Resolves #11409

Summary

This is a follow-up to #6105

#6105 is good but because we don't unify types, some type variables end up unbound. That's why #11409 was failing.

The solution here is:

• To unify the types
• We store the bindings that resulted from that unification. We associate those to the macro expression (an ExprId)
• If we bump into that macro expression again, we first undo those bindings

This allows a variable to change types across macro loop iterations.

For this I also had to relax the checking of unbound generics when a function context is popped, only in comptime code. The reason is that in the new test here, in foo.len() we end up with foo without a known N, but it's fine because it's that N that we want to infer later on. In this case, if N ends up being used when interpreting the block and it's not bound, we'll get an error anyway.

Additional Context

User Documentation

Check one:

• No user documentation needed.
• Changes in docs/ included in this PR.
• [For Experimental Features] Changes in docs/ to be submitted in a separate PR.

PR Checklist

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Test Suite Duration'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: ecabab9	Previous: 8d91700	Ratio
test_report_zkpassport_noir-ecdsa_	3 s	2 s	1.50

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[github-actions]

⚠️ Performance Alert ⚠️

Possible performance regression was detected for benchmark 'Brillig Execution Time'.
Benchmark result of this commit is worse than the previous benchmark result exceeding threshold 1.20.

Benchmark suite	Current: ecabab9	Previous: 8d91700	Ratio
rollup-block-root	0.004 s	0.003 s	1.33

This comment was automatically generated by workflow using github-action-benchmark.

CC: @TomAFrench

[jfecher]

Somewhat of a complex interaction between comptime & the type system but the old behavior with not unifying was also unexpected and this reigns that in.