[Typed throws] Implement conformance checking for typed throws

[DougGregor]

Implement conformance checking for typed throws, including associated type inference, so one can capture the type that is thrown from a witness in a concrete type in an associated type, including any Error (for untyped throws) and Never (for non-throwing functions). For example:

protocol FailureAssociatedType {
  associatedtype Failure: Error

  func f() throws(Failure)
}

struct S1: FailureAssociatedType {
  func f() throws(MyError) { } // infers Failure == MyError
}

struct S2: FailureAssociatedType {
  func f() throws { } // infers Failure == any Error
}

struct S3: FailureAssociatedType {
  func f() { } // infers Failure == Never
}

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please smoke test

[DougGregor]

@swift-ci please test Windows

[DougGregor]

@swift-ci please test Windows