use the correct ParamEnv when checking future's output type

[y21]

Fixes #13862

missing_headers::check is sometimes called from outside of a body (specifically, from check_attributes, where the LateContext's ParamEnv is not yet properly initialized for that item). Using that empty ParamEnv for trait solving things from within the body can then lead to various ICEs, like the linked issue where we have a const generic parameter DMA_INST without a ConstArgHasType bound in the ParamEnv so the const parameter has no type, which is normally not supposed to happen.

We have the item's DefId so we can just get its ParamEnv/TypingEnv from there, and using that one for trait solving should be safe.

changelog: none

[rustbot]

r? @dswij

rustbot has assigned @dswij.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer