After typechk use closure_captures

[arora-aman]

Since upvars_mentioned might not be same as paths that are captured, once type check is done, we know the list of captured path and we should be using that instead.

• librustc_middle/mir
• librustc_mir
• librustc_mir_build
• librustc_passes/liveness.rs

[arora-aman]

change tcx.upvars_mentioned -> tcx.typeck_tables_of(def_id.expect_local()).closure_captures[def_id]

We can only access typechk tables for definitions in the same (i.e. local) crate. So we need to pass the local id to read the typechk tables.

[arora-aman]

Currently closure_captures only provides infromation about the declaration of a captured variable and not the access within the closure. We need information about access to report information like unused variables that was implemented as part of rust-lang/rust#72465.

This is information is not currently avaialble through typechk tables but however when we refector closure_captures to use PlaceUnique will have the hir_id to the access expression.

Waiting on #7 to make more progress.

Zulip stream: https://rust-lang.zulipchat.com/#narrow/stream/189812-t-compiler.2Fwg-rfc-2229/topic/upvar.20access.20spans