GSB: Fix off-by-one error when merging layout requirements of two equivalence classes

Literally an off-by-one error -- we were skipping over the first
requirement and not copying it over. I think this is because the
updateLayout() call used to be addLayoutRequirementDirect(),
which would add the first layout constraint, and I forgot to
remove the '+ 1' when I refactored this.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -5129,7 +5129,7 @@ GenericSignatureBuilder::addSameTypeRequirementBetweenTypeParameters(
       updateLayout(T1, equivClass2->layout);
       equivClass->layoutConstraints.insert(
                                    equivClass->layoutConstraints.end(),
-                                   equivClass2->layoutConstraints.begin() + 1,
+                                   equivClass2->layoutConstraints.begin(),
                                    equivClass2->layoutConstraints.end());
     }
   }

test/Generics/derived_via_concrete.swift

@@ -60,8 +60,14 @@ func derivedViaConcreteZ1<A, B>(_: A, _: B)
 // expected-warning@-1 {{redundant conformance constraint 'A' : 'W'}}
 // expected-note@-2 {{conformance constraint 'A' : 'W' implied here}}
 
+// FIXME: We should not diagnose 'B : AnyObject' as redundant, even
+// though it really is, because it was made redundant by an inferred
+// requirement.
+
 // CHECK: Generic signature: <A, B where A : Z<B>, B : AnyObject>
 func derivedViaConcreteZ2<A, B>(_: A, _: B)
   where A : W, B : AnyObject, A : Z<B> {}
 // expected-warning@-1 {{redundant conformance constraint 'A' : 'W'}}
-// expected-note@-2 {{conformance constraint 'A' : 'W' implied here}}
+// expected-note@-2 {{conformance constraint 'A' : 'W' implied here}}
+// expected-warning@-3 {{redundant constraint 'B' : 'AnyObject'}}
+// expected-note@-4 {{constraint 'B' : 'AnyObject' implied here}}