[len_without_is_empty]: follow type alias to find inherent is_empty method

[y21]

Fixes #11165

When we see an impl B and B is a type alias to some type A, then we need to follow the type alias to look for an is_empty method on the aliased type A. Before this PR, it'd get the inherent impls of B, which there aren't any and so it would warn that there isn't an is_empty method even if there was one.
Passing the type alias DefId to TyCtxt::type_of gives us the aliased DefId (or simply return the type itself if it wasn't a type alias) so we can just use that

changelog: [len_without_is_empty]: follow type alias to find inherent is_empty method

[rustbot]

r? @Centri3

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[Centri3]

Bit weird how inherent_impls doesn't already do this (i.e., strip type aliases), but looks fine to me regardless. Thanks!

@bors r+

[bors]

📌 Commit 26c0f97 has been approved by Centri3

It is now in the queue for this repository.

[bors]

⌛ Testing commit 26c0f97 with merge 2009b58...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Centri3
Pushing 2009b58 to master...