Deduplicate obligations in `opt_normalize_projection_type`#91186

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the8472 wants to merge 3 commits intorust-lang:masterfrom

the8472:simplify-obligation-cause-hash

Commits on Dec 26, 2021

dedup obligations in `opt_normalize_projection_type`
the8472
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don't rebuild Vec when dedup might just shrink it enough that resizing isn't needed
the8472
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Commits on Dec 27, 2021

remove redundant use_cache check already covered by outer conditional
the8472
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