SE-0413: Typed throws

[wadetregaskis]

@xwu

Currently, we disallow (arbitrarily) func f() throws async in favor of async throws. However, if typed throws did not require parentheses, then func f() throws async would become valid Swift, and it would mean a synchronous function that throws an error of type async. This would be the case whether or not you declare such an unhinged type, because it's not up to the grammar what types you declare or not so long as it's a valid identifier.

Admittedly, the compiler could detect this and essentially just ban it, by throwing an error a la "Cannot determine if you meant to make this function async or throw the Error type async (you should probably rename your async struct)".

But there are other potential benefits of the parenthesis, like readability through compartmentalisation. e.g. for throws(some NetworkError), let-alone for less trivial types like throws(some NetworkError & LoggableError & ChainableError).