Handle Foo { a: usize, b: usize } used as Foo(1, 2) gracefully

[estebank]

Given the code

rust/src/test/ui/parser/recover-from-bad-variant.rs

Lines 1 to 14 in 3592079

	enum Enum {
	Foo { a: usize, b: usize },
	Bar(usize, usize),
	}
	fn main() {
	let x = Enum::Foo(a: 3, b: 4);
	//~^ ERROR expected type, found `3`
	match x {
	Enum::Foo(a, b) => {}
	//~^ ERROR expected tuple struct/variant, found struct variant `Enum::Foo`
	Enum::Bar(a, b) => {}
	}
	}

we emit

rust/src/test/ui/parser/recover-from-bad-variant.stderr

Lines 1 to 21 in 3592079

	error: expected type, found `3`
	--> $DIR/recover-from-bad-variant.rs:7:26
	|
	LL | let x = Enum::Foo(a: 3, b: 4);
	| ^ expecting a type here because of type ascription
	|
	= note: type ascription is a nightly-only feature that lets you annotate an expression with a type: `<expr>: <type>`
	note: this expression expects an ascribed type after the colon
	--> $DIR/recover-from-bad-variant.rs:7:23
	|
	LL | let x = Enum::Foo(a: 3, b: 4);
	| ^
	= help: this might be indicative of a syntax error elsewhere
	error[E0532]: expected tuple struct/variant, found struct variant `Enum::Foo`
	--> $DIR/recover-from-bad-variant.rs:10:9
	|
	LL | Enum::Foo(a, b) => {}
	| ^^^^^^^^^ did you mean `Enum::Foo { /* fields */ }`?
	error: aborting due to 2 previous errors

We should emit something closer to

error: expected type, found `3`
  --> $DIR/recover-from-bad-variant.rs:7:26
   |
LL |     let x = Enum::Foo(a: 3, b: 4);
   |                          ^ expecting a type here because of type ascription

error[E0532]: expected tuple struct/variant, found struct variant `Enum::Foo`
  --> $DIR/recover-from-bad-variant.rs:10:9
   |
LL |         Enum::Foo(a, b) => {}
   |         ^^^^^^^^^ did you mean `Enum::Foo { a, b }`?

error: aborting due to 2 previous errors

[estebank]

Reopening to track the let x = Enum::Foo(a: 3, b: 4); case.