Disallow cast with trailing braced macro in let-else

[dtolnay]

This fixes an edge case I noticed while porting #118880 and #119062 to syn.

Previously, rustc incorrectly accepted code such as:

let foo = &std::ptr::null as &'static dyn std::ops::Fn() -> *const primitive! {
    8
} else {
    return;
};

even though a right curl brace } directly before else in a let...else statement is not supposed to be valid syntax.