Auto merge of #60840 - tmandry:preserve-scope-in-generator-mir, r=cramertj

Preserve local scopes in generator MIR

Part of #52924, depended upon by the generator layout optimization #60187.

This PR adds `StorageDead` statements in more places in generators, so we can see when non-`Drop` locals have gone out of scope and recover their storage.

The reason this is only done for generators is compiler performance. See https://github.com/rust-lang/rust/pull/60187#issuecomment-485637811 for what happens when we do this for all functions.

For `Drop` locals, we modify the `MaybeStorageLive` analysis to use `drop` to indicate that storage is no longer live for the local. Once `drop` returns or unwinds to our function, we implicitly assume that the local is `StorageDead`.

Instead of using `drop`, it is possible to emit more `StorageDead` statements in the MIR for `Drop` locals so we can handle all locals the same. I am fine with doing it that way, but this was the simplest approach for my purposes. It is also likely to be more performant.

r? @Zoxc (feel free to reassign)
cc @cramertj @eddyb @RalfJung @rust-lang/wg-async-await

src/test/mir-opt/generator-drop-cleanup.rs

@@ -17,6 +17,7 @@ fn main() {
 //     switchInt(move _5) -> [0u32: bb4, 3u32: bb7, otherwise: bb8];
 // }
 // bb1: {
+//     StorageDead(_3);
 //     goto -> bb5;
 // }
 // bb2: {