swift234

Produce an error for generic parameters of functions if those parameters are not used in function signature.

If a generic parameter is not referred to from a function signature, it can never be inferred and thus such a function can never be invoked.

We now produce the following error:

generic parameter 'T' is not used in function signature
func f8<T> (x: Int) {}

This commit takes Jordan't comments on r28181 into account:
- it produces a shorter error message
- it does not change the compiler_crashers_fixed test and add a new expected error instead

Swift SVN r28194