Issue Description:

In function declarations, the parser looks for a throws after the -> token instead of before it; if it finds one, it diagnoses a special error with a fix-it swapping the two tokens. But if you make the same mistake in a function type, Swift shows a series of mildly confusing error messages like “expected type for function result”. Function types should provide a similar experience to function declarations.

The code that does this for function declarations is in a method called parseFunctionSignature(). The test case for this feature would look something like this (although the fix-it might actually be done slightly differently):

let fn: () -> throws Void  // expected-error{{‘throws’ may only occur before ‘->’}} {{12-12=throws }} {{15-22=}}