Move Upvar lowering from THIR to MIR

[arora-aman]

We lower paths that start from an Upvar here.

Essentially what this code does is convert upvars to refer to a field within the desugared ClosureSubsts. We want to refactor this code as follows:

• Instead of doing the above desugaring we want to return Thir::Expr::UpvarRef(var_hir_id) in case the var_hir_id is mentioned within the closure. (This will allow us to deal with the let _ = x problem in MIR).
• Move the logic currently in convert_var to rustc_mir_build/as_place
• Maybe remove Thir::Expr::SelfRef, it's most likely only for Closures.