Update PlaceTy to use ty::Variant

[zhamlin]

Part 1 of the plan defined here

[rust-highfive]

Some changes occured to the CTFE / Miri engine

cc @rust-lang/miri

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @matthewjasper (or someone else) soon.

Please see the contribution instructions for more information.

[oli-obk]

We should document each such use to explain why it doesn't matter. E.g. this case is because we only care about whether the type has a drop impl itself. Though we could just add a method to Ty that returns whether it has a drop impl and make that function work for both variant and adt.

[zhamlin]

I went with the adding a method to Ty approach, seemed better than using strip_variant_type here.

[oli-obk]

I didn't go through the unimplemented! calls yet

[zhamlin]

For the unimplemented!, I can go ahead and update them to use recursion where possible.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

    Checking tracing-tree v0.1.9
    Checking rustdoc-json-types v0.1.0 (/checkout/src/rustdoc-json-types)
    Checking tera v1.10.0
    Checking rustdoc v0.0.0 (/checkout/src/librustdoc)
error[E0004]: non-exhaustive patterns: `Variant(_, _)` not covered
    --> src/librustdoc/clean/mod.rs:1396:15
     |
1396 |         match *ty.kind() {
     |               ^^^^^^^^^^ pattern `Variant(_, _)` not covered
    ::: /checkout/compiler/rustc_middle/src/ty/sty.rs:199:5
     |
     |
199  |     Variant(Ty<'tcx>, VariantIdx),
     |
     = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
     = note: the matched value is of type `rustc_middle::ty::TyKind`


error[E0004]: non-exhaustive patterns: `Variant(_, _)` not covered
    |
    |
539 |         Some(match *self.cx.tcx.type_of(ty_id).kind() {
    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ pattern `Variant(_, _)` not covered
   ::: /checkout/compiler/rustc_middle/src/ty/sty.rs:199:5
    |
    |
199 |     Variant(Ty<'tcx>, VariantIdx),
    |
    = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
    = note: the matched value is of type `rustc_middle::ty::TyKind`