Preserve placeholder ambiguity during normalization

[amirHdev]

Fixes #159896

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r? @chenyukang

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[chenyukang]

@rustbot reroll

[petrochenkov]

r? types

[lcnr]

I immediately get this change, can you provide some comment/PR description for what went wrong and why this is correct

r? BoxyUwU

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Failed to set assignee to BoxyUwU: error sending request

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[amirHdev]

I immediately get this change, can you provide some comment/PR description for what went wrong and why this is correct

The problem was that the certainty of the nested goals was combined with the placeholder certainty and then dropped in the normalization path. It is fine to ignore ambiguity from nested goals there since those goals are returned to the caller. placeholder ambiguity is different because it is not reflected in the returned goals
this patch keeps the two values separate and only ignores the nested goal certainty

[lcnr]

r? lcnr makes sense

@bors r+ rollup

[rust-bors]

📌 Commit ba4e2f6 has been approved by lcnr

It is now in the queue for this repository.

[jhpratt]

@bors r- #160128 (comment)

[rust-bors]

This pull request was unapproved.

This PR was contained in a rollup (#160128), which was unapproved.

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This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

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[amirHdev]

@lcnr Rebased onto current main and fixed the ICE by restoring normalization of TypeOutlives caller bounds
@rustbot ready

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