Rollup merge of #141811 - mejrs:bye_locals, r=compiler-errors

Unimplement unsized_locals

Implements https://github.com/rust-lang/compiler-team/issues/630

Tracking issue here: https://github.com/rust-lang/rust/issues/111942

Note that this just removes the feature, not the implementation, and does not touch `unsized_fn_params`. This is because it is required to support `Box<dyn FnOnce()>: FnOnce()`.

There may be more that should be removed (possibly in follow up prs)
- the `forget_unsized` function and `forget` intrinsic.
- the `unsized_locals` test directory; I've just fixed up the tests for now
- various codegen support for unsized values and allocas

cc ``@JakobDegen`` ``@oli-obk`` ``@Noratrieb`` ``@programmerjake`` ``@bjorn3``

``@rustbot`` label F-unsized_locals

Fixes rust-lang/rust#79409

compiler/rustc_feature/src/unstable.rs

@@ -667,8 +667,6 @@ declare_features! (
     (incomplete, unsized_const_params, "1.82.0", Some(95174)),
     /// Allows unsized fn parameters.
     (internal, unsized_fn_params, "1.49.0", Some(48055)),
-    /// Allows unsized rvalues at arguments and parameters.
-    (incomplete, unsized_locals, "1.30.0", Some(48055)),
     /// Allows using the `#[used(linker)]` (or `#[used(compiler)]`) attribute.
     (unstable, used_with_arg, "1.60.0", Some(93798)),
     /// Allows use of attributes in `where` clauses.