Custom diagnostic for bad mut in pattern with sub-bindings

[estebank]

Given (as brought up in #63693 (comment) CC @Centril):

pub fn main() {
    struct Foo { x: isize }
    let mut Foo { x: x } = Foo { x: 3 };
}

We currently emit a parse error:

error: expected one of `:`, `;`, `=`, or `@`, found `{`
 --> src/main.rs:3:17
  |
3 |     let mut Foo { x: x } = Foo { x: 3 };
  |                 ^ expected one of `:`, `;`, `=`, or `@` here

It should instead parse as if it were let Foo { x: mut x } = Foo { x: 3 }; (medium priority), silence any complaints that mut is unnecessary (lower priority) and emit a custom error explaining that mut cannot lead a pattern with subpatterns (must have), and suggest the appropriate mut placement in the sub-bindings (nice to have):

error: incorrect `mut` pattern
 --> src/main.rs:3:17
  |
3 |     let mut Foo { x: x } = Foo { x: 3 };
  |         ^^^ patterns with sub-bindings can't be marked as `mut`
help: mark the individual sub-bindings as `mut` instead
3 |     let Foo { x: mut x } = Foo { x: 3 };
  |        --        ^^^

[Centril]

This should be relatively easy to do by hooking into

rust/src/libsyntax/parse/parser/pat.rs

Line 289 in bea0372

fn recover_pat_ident_mut_first(&mut self) -> PResult<'a, PatKind> {

or where it is called. If a ref token does not follow you simply call the recursive pattern parser and then once you have parsed a PatKind you walk (with a mutating visitor) to all the binding sites, insert a mut there, pretty print it and suggest the output.

[Centril]

Marking as blocked for now on #63693 so we don't get a lot of conflicts.