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

Unimplement unsized_locals

Implements rust-lang/compiler-team#630

Tracking issue here: rust-lang/rust#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

Author: matthiaskrgr