Add documentation to the cast optimizer change

Author: aschwaighofer

lib/SILOptimizer/Utils/CastOptimizer.cpp

@@ -1434,6 +1434,8 @@ static bool optimizeStaticallyKnownProtocolConformance(
                                                       SourceType, Conformances);
           auto Projection =
               B.createProjectExistentialBox(Loc, Src->getType(), AllocBox);
+          // This needs to be a copy_addr (for now) because we must handle
+          // address-only types.
           B.createCopyAddr(Loc, Src, Projection, IsTake, IsInitialization);
           B.createStore(Loc, AllocBox, Dest,
                         swift::StoreOwnershipQualifier::Unqualified);

test/SILOptimizer/sil_combine_objc_bridge.sil

@@ -202,6 +202,8 @@ enum AddressOnlyError : Error {
   case none
 }
 
+// Test that we emit a copy_addr for the address-only value initialization.
+
 // CHECK-LABEL: sil @unconditional_checked_cast_addr_address_only_type
 // CHECK: alloc_stack $Error
 // CHECK: alloc_stack $AddressOnlyError