Fix i14451

[Sporarum]

Fixes #14451

Implicitly assumed type clauses could only be at the beginning, which is wrong since:

extension (x: Int) def foo[T](y: T) = ???

de-sugars to something like:

def foo(x: Int)[T](y: T) = ???

To fix it, I implement stripInferrable, a variant of stripImplicit which also drops type clauses, and use it in resultIsMethod
I suspect the other uses of stripImplicit could be simplified, or even fixed (assuming they make the same mistake as resultIsMethod), by using stripInferrable

[sjrd]

AFAICT, this looks good to me.