SIL: Workaround for GenericSignatureBuilder bug

The GSB will drop same-type requirements sometimes, when the
right hand side is smaller than the left. Change the order
when adding these requirements, which seems to work well
enough for my reduced testcase.

Also, ensure that everything works correctly with the
RequirementMachine, which doesn't have the same underlying
problems with same-type requirement handling.

Fixes rdar://86431977.

Author: slavapestov

lib/SIL/IR/AbstractionPattern.cpp

@@ -1649,8 +1649,8 @@ class SubstFunctionTypePatternVisitor
       newGPMapping.insert({gp, newParamTy});
       auto substGPTy = Type(gp).subst(substGPMap)->castTo<GenericTypeParamType>();
       substRequirements.push_back(Requirement(RequirementKind::SameType,
-                                              substGPTy,
-                                              newParamTy));
+                                              newParamTy,
+                                              substGPTy));
       assert(!substReplacementTypes[substGPTy->getIndex()]);
       substReplacementTypes[substGPTy->getIndex()] = substParamTy;
     }

test/SILGen/type_lowering_subst_function_type_requirement_machine.swift

@@ -0,0 +1,22 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+struct G<Key: CaseIterable, Value> where Key: RawRepresentable, Value: Codable {
+  var key: Key.RawValue
+}
+
+protocol P: CaseIterable, RawRepresentable {}
+
+struct Value: Codable {}
+
+enum Key: Int, P {
+  case elt
+}
+
+func callee<Key: P>(_: Key.Type, _: @escaping (G<Key, Value>) -> Void) {}
+
+// CHECK-LABEL: sil hidden [ossa] @$s029type_lowering_subst_function_A20_requirement_machine6calleryyF : $@convention(thin) () -> () {
+// CHECK: function_ref @$s029type_lowering_subst_function_A20_requirement_machine6calleryyFyAA1GVyAA3KeyOAA5ValueVGcfU_ : $@convention(thin) @substituted <τ_0_0, τ_0_1, τ_0_2 where τ_0_0 : CaseIterable, τ_0_0 : RawRepresentable, τ_0_0 == τ_0_2, τ_0_1 == Value> (@in_guaranteed G<τ_0_0, Value>) -> () for <Key, Value, Key>
+func caller() {
+  callee(Key.self, { _ in })
+}
+