Improve "no type for local variable" error when in check_const_with_ty

[pnkfelix]

The control flow for rustc_typeck::check::check_const_with_ty can end up spitting out an error of the form:

non-constant-expr-for-fixed-len-vec.rs:15:25: 15:26 error: no type for local variable 10
non-constant-expr-for-fixed-len-vec.rs:15         let _x: [isize; n];

With the compiler as it stands today, this is usually coupled with an error of the form:

error: array length constant evaluation error: non-constant path in constant expr

but other changes being made may lead to only the "no type for local variable" message being emitted.

We can do better here, e.g. by passing along knowledge that we are looking at a const-expr and then reporting that local variables simply are invalid within const-exprs.

[dgopstein]

Just received a lone no type for local variable. It's reproducible with the following program:

fn main() {
  let len: usize = 1;
  let arr: [char; len] = ['a'];
}

[pyk]

Any update for more informative error message? as described in #31082

[eddyb]

This is an outright bug, the correct behavior is for the use of a variable from a constant to not resolve.
Fixed on an in-progress branch, self-assigning for tracking.