[GSB] "Self: P" in requirement signatures aren't self-derived.

When determining whether a conformance requirement source is
self-derived, ignore the top-level "Self: P" requirement in a
requirement signature.

Fixes SR-5485 / rdar://problem/33360699.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -419,6 +419,11 @@ bool RequirementSource::isSelfDerivedConformance(
     switch (source->kind) {
     case ProtocolRequirement:
     case InferredProtocolRequirement: {
+      // For a requirement signature, we ignore the 'Self: P' requirement
+      // for the purposes of the self-derived check.
+      if (source->parent->kind == RequirementSource::RequirementSignatureSelf)
+        return false;
+
       // Note that we've seen a protocol requirement.
       sawProtocolRequirement = true;
 

test/decl/protocol/recursive_requirement_ok.swift

@@ -12,3 +12,12 @@ func testP<T: P>(_ t: T) {
   testP(t.assoc.assoc.assoc)
   testP(t.assoc.assoc.assoc.assoc.assoc.assoc.assoc)
 }
+
+// SR-5485
+protocol P1 {
+  associatedtype X : P2
+}
+protocol P2 {
+  associatedtype Y : P1 where Y.X == Self // expected-note{{conformance constraint 'Self.Z': 'P1' implied here}}
+  associatedtype Z : P1 // expected-warning{{redundant conformance constraint 'Self.Z': 'P1'}}
+}