Silence redundant failed obligations on the same statement

[estebank]

Fix #84346, fix #105753.

On multiple unment obligations in the same method chain, only emit one. On unmet Sized obligations on a binding, its type and its init expression, only emit one.

[rustbot]

changes to the core type system

cc @lcnr

[rustbot]

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[estebank]

Easier to review per-comit: first add mild mechanic changes to have more tracking to be used later, added the tests that I wanted to tackle, and finally the actual logic change that affects the emitted errors.

[oli-obk]

may be worth figuring out a WellFormedLoc for, as this is the only use of None that won't be removed when the old solver gets nuked Soon ™️

View changes since the review

[estebank]

Using the HirId of the expr doesn't change anything today (because it is the offset_of! macro which already points at the whole expr), but might in the future. Changed in the last commit.

[rustbot]

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