[GSB] Only check protocol requirements not generated from the signature.

If we used the requirement signature to create a protocol requirement
element in a requirement source, there's no need to verify that it's
from the requirement signature (duh).

Author: DougGregor

include/swift/AST/GenericSignatureBuilder.h

@@ -936,9 +936,11 @@ class GenericSignatureBuilder::RequirementSource final
   /// Whether there is a trailing written requirement location.
   const bool hasTrailingWrittenRequirementLoc;
 
+public:
   /// Whether a protocol requirement came from the requirement signature.
   const bool usesRequirementSignature;
 
+private:
   /// The actual storage, described by \c storageKind.
   union {
     /// The type to which a requirement applies.

lib/AST/GenericSignature.cpp

@@ -939,7 +939,8 @@ static bool hasNonCanonicalSelfProtocolRequirement(
     }
 
     // Check whether the given requirement is in the requirement signature.
-    if (!hasConformanceInSignature(inProto->getRequirementSignature(),
+    if (!source->usesRequirementSignature &&
+        !hasConformanceInSignature(inProto->getRequirementSignature(),
                                    source->getStoredType(), conformingProto))
       return true;