[GSB] Ensure that we have superclass constraints from merged equiv class.

Fixes one recently-found crasher.

(cherry picked from commit e256a9d)
(cherry picked from commit f72fec0)

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -3391,12 +3391,13 @@ GenericSignatureBuilder::addSameTypeRequirementBetweenArchetypes(
     else
       source2 = equivClass2->superclassConstraints.front().source;
 
-    (void)updateSuperclass(T1, equivClass2->superclass, source2);
-
+    // Add the superclass constraints from the second equivalence class.
     equivClass->superclassConstraints.insert(
                                    equivClass->superclassConstraints.end(),
                                    equivClass2->superclassConstraints.begin(),
                                    equivClass2->superclassConstraints.end());
+
+    (void)updateSuperclass(T1, equivClass2->superclass, source2);
   }
 
   // Add all of the protocol conformance requirements of T2 to T1.

validation-test/compiler_crashers/28758-swift-genericsignaturebuilder-resolvesuperconformance-swift-genericsignaturebuil.swift renamed to validation-test/compiler_crashers_fixed/28758-swift-genericsignaturebuilder-resolvesuperconformance-swift-genericsignaturebuil.swift

@@ -5,5 +5,5 @@
 // See https://swift.org/LICENSE.txt for license information
 // See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
 
-// RUN: not --crash %target-swift-frontend %s -emit-ir
+// RUN: not %target-swift-frontend %s -emit-ir
 protocol P{{}typealias e:a{}}class a:P=extension P{typealias e:Self