use the correct `ParamEnv` when checking future's output type (#13863)

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

0 files changed, 0 insertions, 0 deletions