Mention expectation of `{` for parse error immediately after `async` in expressions

[estebank]

The following code

rust/src/test/ui/parser/block-no-opening-brace.stderr

Lines 34 to 40 in fe531d5

	error: expected one of `move`, `|`, or `||`, found keyword `let`
	--> $DIR/block-no-opening-brace.rs:30:9
	|
	LL | async
	| - expected one of `move`, `|`, or `||`
	LL | let x = 0;
	| ^^^ unexpected token

should also mention an expectation for an open brace {, and potentially even suggesting a new block, like the other test cases in that file.