GSB: Assert that a minimal signature doesn't have protocols refined by other protocols

I'm looking at a bug where we end with a signature like

    protocol B { ... }
    protocol BB : B { }

    <Self where Self : B, Self : BB>

While this one be a one-off bug, it's easy enough to check for this
condition with an assert here.

lib/AST/GenericSignatureBuilder.cpp

@@ -7997,6 +7997,9 @@ static void checkGenericSignature(CanGenericSignature canSig,
 
   auto canonicalRequirements = canSig.getRequirements();
 
+  // We collect conformance requirements to check that they're minimal.
+  llvm::SmallDenseMap<CanType, SmallVector<ProtocolDecl *, 2>, 2> conformances;
+
   // Check that the signature is canonical.
   for (unsigned idx : indices(canonicalRequirements)) {
     debugStack.setRequirement(idx);
@@ -8047,6 +8050,10 @@ static void checkGenericSignature(CanGenericSignature canSig,
              "Left-hand side must be a type parameter");
       assert(isa<ProtocolType>(reqt.getSecondType().getPointer()) &&
              "Right-hand side of conformance isn't a protocol type");
+
+      // Collect all conformance requirements on each type parameter.
+      conformances[CanType(reqt.getFirstType())].push_back(
+          reqt.getProtocolDecl());
       break;
     }
 
@@ -8089,6 +8096,22 @@ static void checkGenericSignature(CanGenericSignature canSig,
     assert(prevReqt.compare(reqt) < 0 &&
            "Out-of-order requirements");
   }
+
+  // Make sure we don't have redundant protocol conformance requirements.
+  for (auto pair : conformances) {
+    const auto &protos = pair.second;
+    auto canonicalProtos = protos;
+
+    // canonicalizeProtocols() will sort them and filter out any protocols that
+    // are refined by other protocols in the list. It should be a no-op at this
+    // point.
+    ProtocolType::canonicalizeProtocols(canonicalProtos);
+
+    assert(protos.size() == canonicalProtos.size() &&
+           "redundant conformance requirements");
+    assert(std::equal(protos.begin(), protos.end(), canonicalProtos.begin()) &&
+           "out-of-order conformance requirements");
+  }
 }
 #endif