Ban old-style type assertions as arrow function bodies under erasableSyntaxOnly

[jakebailey]

Code like ()=><any>{} is not erasable, since ()=> {} is a different tree, and there's nowhere to add parens to the body without shifting its contents backwards one character. If there were extra spaces after the {}, maybe it'd work, but that's atypical.

This PR bans this specific case, though, I personally feel like banning the old-style assertions would be a good idea too.

[ajafff]

you probably want to ban it in more places:

// return and yield ASI
function *foo() {
    yield <any>
        1;
    return <any>
        1;
}

// at the start of an ExpressionStatement if followed by an object literal; though I'm not sure why one would use it there
<unknown>{foo() {}}.foo();

// at the start of an ExpressionStatement if followed by function keyword
<unknown>function() {}();
<unknown>function() {};

// at the start of an ExpressionStatement if followed by an anonymous class expression
// note that this exact syntax currently emits invalid JS (no parenthesis added like for function above)
<unknown>class {}

there's probably more parsing ambiguity I cannot remember right now.

[jakebailey]

Yeah, so that furthers my feeling that this syntax should be wholly banned in this mode. I tried to carve something out, but it sure doesn't seem like that's tractable if we want to ban something here.