Support function args const generics

[bit-aloo]

This PR adds initial support for argument-position const generics behind the function_arg_const_generics feature gate.

It allows const generics to be declared directly in function arguments. The PR adds the basic plumbing across AST, HIR, and ty, parses these parameters into the function's generics, lowers call arguments into consts. For now, we lower literals, negated literals, const parameters, const items, unit variants, and etc, This works for functions, methods and a couple of more cases.

r? @BoxyUwU
cc: @khyperia

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[rustbot]

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

cc @rust-lang/rustfmt

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

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

cc @fmease

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

HIR ty lowering was modified

cc @fmease

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

cc @rust-lang/clippy

[ytmimi]

What is arg_pos, and does rustfmt need to handle formatting it in some way?

Probably good to add a #![feature(function_arg_const_generics] test case to rustfmt to make sure things are getting formatted as expected.

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