We were redundantly printing constraints that were implied by the base
type (the type being extended). Rather than special-casing constraints
on a protocol's 'Self' type, omit any requirements that are already
satisfied by the extended type instead.
https://bugs.swift.org/browse/SR-7413
For instance:
protocol P1 {
func foo()
}
protocol P2 : P1 {
func bar()
}
extension P2 {
func foo() {}
}
We report the foo() in P2's extension as the default implementation of foo() declared in P1.
