[Generic Signature Builder] Sort invalid source locations properly.

Fixes SR-9496 / rdar://problem/46699008.

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -6831,6 +6831,14 @@ void GenericSignatureBuilder::checkSameTypeConstraints(
           auto locA = (*a)->constraint.source->getLoc();
           auto locB = (*b)->constraint.source->getLoc();
 
+          // Put invalid locations after valid ones.
+          if (locA.isInvalid() || locB.isInvalid()) {
+            if (locA.isInvalid() != locB.isInvalid())
+              return locA.isInvalid() ? 1 : -1;
+
+            return 0;
+          }
+
           auto bufferA = sourceMgr.findBufferContainingLoc(locA);
           auto bufferB = sourceMgr.findBufferContainingLoc(locB);
 

validation-test/compiler_crashers_2_fixed/0188-sr9496.swift

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend -typecheck %s
+
+protocol P1 {
+    associatedtype A1
+}
+
+protocol P2 {
+    associatedtype A2
+}
+
+struct S1<G1: P1, G2: P1>: P1 where G1.A1 == G2.A1 {
+    typealias A1 = G1.A1
+}
+
+struct S2<G1: P1, G2: P2>: P2 where G1.A1 == G2.A2 {
+    typealias A2 = G2.A2
+}
+
+struct S3<G1: P1, G2: P2> where G1.A1 == G2.A2 {
+    func f<G: P1>(_: G) -> S3<S1<G, G1>, S2<G, G2>> {
+        fatalError()
+    }
+}