Value can not be on the left-hand side of += or friends

[xffxff]

#83 added support for += and friends (see #69), but value can not be on the left-hand side of +=.
The following test should get a validation error, as foo() is a value, not a place in memory

fn foo() {
    1
}

async fn main() {
    foo() += 2
}

[nikomatsakis]

(Note: I think it can get a validation error but continue to execute and compile just as it does not; I assume that it compiles to a temporary which gets modified.)

[Aadamandersson]

I think this can be closed.
It looks like a validation error was added for this in #148. (Commit: #e05bd7166ad7b380cd171fc2f7b8666343bf65ad)

dada/components/dada-validate/src/validate/validator.rs

Lines 695 to 703 in 36b611e

	_ => {
	let _ = self.validate_expr(expr);
	let ErrorReported = dada_ir::error!(
	self.span(expr),
	"you can only assign to local variables and fields, not arbitrary expressions",
	)
	.emit(self.db);
	self.add(validated::ExprData::Error, expr)
	}