[WIP] [DO NOT MERGE] combine #66020 and #66821.#66838

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eddyb wants to merge 2 commits intorust-lang:masterfrom eddyb:wf-skip-plus-global-caching

Commits on Nov 27, 2019

[WIP] rustc: experiment with ParamEnv-aware caching in traits::select.
eddyb
committed

Commits on Nov 28, 2019

rustc: avoid checking Trait's predicates for WF(<T as Trait>::X).
eddyb
committed