Provide more helpful error message when trying to use const generic defaults

[XAMPPRocky]

Defaults are currently not implemented in const generics. With min_const_generics stabilising soon, I've been using them more and one thing that is surprising is the lack of defaults. Especially when you try to declare a default, the error message provides a unexpected syntax error, when I would expect a "defaults in const generics is not implemented" error, leading you to think that there's another syntax that you need to use as opposed to it just not being available.

#![feature(min_const_generics)]

fn foo<const NUM: usize = 0>() -> usize {
    NUM
}

Error Message

error: expected one of `!`, `(`, `+`, `,`, `::`, `<`, or `>`, found `=`
 --> src/lib.rs:3:25
  |
3 | fn foo<const NUM: usize = 0>() -> usize {
  |                         ^ expected one of 7 possible tokens

error: aborting due to previous error

[varkor]

Perhaps the simplest solution is to extract the parsing for const generic defaults from #75384, but unconditionally emit an error in ast_validation.rs if they're used. I think it's very likely we'll want to support const generic defaults before too long, so hopefully the solution is going to be temporary anyway.

cc @JulianKnodt in case you're interested in implementing this, from your experience with #75384.