[Serialization] Drop support for serializing LValueType. (#9080)

This type never appears in API boundaries or in SIL instructions,
which means it never gets serialized.

Author: jrose-apple

include/swift/Serialization/DeclTypeRecordNodes.def

@@ -87,7 +87,7 @@ TYPE(TUPLE)
 TRAILING_INFO(TUPLE_TYPE_ELT)
 TYPE(FUNCTION)
 TYPE(METATYPE)
-TYPE(LVALUE)
+TYPE(LVALUE_unused)
 TYPE(INOUT)
 TYPE(ARCHETYPE)
 TYPE(PROTOCOL_COMPOSITION)

include/swift/Serialization/ModuleFormat.h

@@ -635,11 +635,6 @@ namespace decls_block {
     MetatypeRepresentationField        // representation
   >;
 
-  using LValueTypeLayout = BCRecordLayout<
-    LVALUE_TYPE,
-    TypeIDField // object type
-  >;
-
   using InOutTypeLayout = BCRecordLayout<
     INOUT_TYPE,
     TypeIDField // object type

lib/Serialization/Deserialization.cpp

@@ -3841,15 +3841,9 @@ Expected<Type> ModuleFile::getTypeChecked(TypeID TID) {
     break;
   }
 
-  case decls_block::LVALUE_TYPE: {
-    TypeID objectTypeID;
-    decls_block::LValueTypeLayout::readRecord(scratch, objectTypeID);
-    typeOrOffset = LValueType::get(getType(objectTypeID));
-    break;
-  }
   case decls_block::INOUT_TYPE: {
     TypeID objectTypeID;
-    decls_block::LValueTypeLayout::readRecord(scratch, objectTypeID);
+    decls_block::InOutTypeLayout::readRecord(scratch, objectTypeID);
 
     auto objectTy = getTypeChecked(objectTypeID);
     if (!objectTy)

lib/Serialization/Serialization.cpp

@@ -3449,14 +3449,6 @@ void Serializer::writeType(Type ty) {
     break;
   }
 
-  case TypeKind::LValue: {
-    auto lValueTy = cast<LValueType>(ty.getPointer());
-
-    unsigned abbrCode = DeclTypeAbbrCodes[LValueTypeLayout::Code];
-    LValueTypeLayout::emitRecord(Out, ScratchRecord, abbrCode,
-                                 addTypeRef(lValueTy->getObjectType()));
-    break;
-  }
   case TypeKind::InOut: {
     auto iotTy = cast<InOutType>(ty.getPointer());
 
@@ -3506,6 +3498,8 @@ void Serializer::writeType(Type ty) {
     break;
   }
 
+  case TypeKind::LValue:
+    llvm_unreachable("lvalue types are only used in function bodies");
   case TypeKind::TypeVariable:
     llvm_unreachable("type variables should not escape the type checker");
   }
@@ -3524,7 +3518,6 @@ void Serializer::writeAllDeclsAndTypes() {
   registerDeclTypeAbbr<FunctionTypeLayout>();
   registerDeclTypeAbbr<MetatypeTypeLayout>();
   registerDeclTypeAbbr<ExistentialMetatypeTypeLayout>();
-  registerDeclTypeAbbr<LValueTypeLayout>();
   registerDeclTypeAbbr<InOutTypeLayout>();
   registerDeclTypeAbbr<ArchetypeTypeLayout>();
   registerDeclTypeAbbr<ProtocolCompositionTypeLayout>();