don't ignore ambiguity during MIR type check

[lcnr]

fixes #164054

this was previously fixed in #116530 and later reverted in #118736.

It impacts the new solver more than the old one as the new solver depends significantly more on region identity, see the relevant stabilization report section.

The old solver has significantly less region identity dependence. It does rely on region identity when merging candidates during normalization:

rust/compiler/rustc_trait_selection/src/traits/project.rs

Lines 103 to 107 in 69bccf0

	// Duplicates can happen inside ParamEnv. In the case, we
	// perform a lazy deduplication.
	if current == &candidate {
	return false;
	}

This check results in additional ICE with the old solver while instead triggering the potentially region dependent goals check with the new one

rust/compiler/rustc_trait_selection/src/traits/query/type_op/custom.rs

Lines 94 to 95 in 14f0018

	let value = op(&ocx).map_err(|_| {
	infcx.tcx.check_potentially_region_dependent_goals(root_def_id).err().unwrap_or_else(

r? types

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