[NFC] Added Xllvm flag to control TypeLowering.

The new flag allows TypeLowering to be forced to behave as though
-enable-sil-opaque-values was passed.  That's needed in order to
reasonably write tests on SIL which _notionally_ started out in opaque
values mode and have since been address lowered.

Author: nate-chandler

lib/SIL/IR/TypeLowering.cpp

@@ -43,6 +43,15 @@
 using namespace swift;
 using namespace Lowering;
 
+// Necessary to straightforwardly write SIL tests that exercise
+// OpaqueValueTypeLowering (and MoveOnlyOpaqueValueTypeLowering): the tests can
+// be written as though opaque values were enabled to begin but have since been
+// lowered out of.
+llvm::cl::opt<bool> TypeLoweringForceOpaqueValueLowering(
+    "type-lowering-force-opaque-value-lowering", llvm::cl::init(false),
+    llvm::cl::desc("Force TypeLowering to behave as if building with opaque "
+                   "values enabled"));
+
 namespace {
   /// A CRTP type visitor for deciding whether the metatype for a type
   /// is a singleton type, i.e. whether there can only ever be one
@@ -2066,7 +2075,8 @@ namespace {
 
     TypeLowering *handleMoveOnlyAddressOnly(CanType type,
                                             RecursiveProperties properties) {
-      if (!TC.Context.SILOpts.EnableSILOpaqueValues) {
+      if (!TC.Context.SILOpts.EnableSILOpaqueValues &&
+          !TypeLoweringForceOpaqueValueLowering) {
         auto silType = SILType::getPrimitiveAddressType(type);
         return new (TC)
             MoveOnlyAddressOnlyTypeLowering(silType, properties, Expansion);
@@ -2084,7 +2094,8 @@ namespace {
 
     TypeLowering *handleAddressOnly(CanType type,
                                     RecursiveProperties properties) {
-      if (!TC.Context.SILOpts.EnableSILOpaqueValues) {
+      if (!TC.Context.SILOpts.EnableSILOpaqueValues &&
+          !TypeLoweringForceOpaqueValueLowering) {
         auto silType = SILType::getPrimitiveAddressType(type);
         return new (TC) AddressOnlyTypeLowering(silType, properties,
                                                            Expansion);