remove MutTy

[cyrgani]

This PR removes ast::MutTy and hir::MutTy and inlines their two fields directly into their corresponding TypeKind variants. These types probably made sense in pre-1.0 versions where it still had a syntax-level representation with things like [mut T]. Nowadays, it is just a type that is used in some, but not all places to group a type and a mutability together (for example, rustc_type_ir does not include such a wrapper). It also has no methods and just makes some match statements more verbose. Removing it makes them more readable.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

rustfmt is developed in its own repository. If possible, consider making this change to rust-lang/rustfmt instead.

cc @rust-lang/rustfmt

Some changes occurred in compiler/rustc_builtin_macros/src/autodiff.rs

cc @ZuseZ4

HIR ty lowering was modified

cc @fmease

Some changes occurred in compiler/rustc_ast/src/expand/autodiff_attrs.rs

cc @ZuseZ4

[rustbot]

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[oli-obk]

Huh I thought we had this before and changed at some point to bundle the pointer with its mutability. Or was that only in TyKind?

Either way, we should keep all three in sync, so also do the change for TyKind::Ref and Ptr, or keep all three in the MutTy system.

[cyrgani]

It was removed from rustc_type_ir with #122852 and rust-lang/types-team#124.