Temporarily add EmptyPass to prevent glslang from failing (#4004)

Removing PropagateLineInfoPass and RedundantLineInfoElimPass from
56d0f5035 makes unit tests of many open source projects fail.
It will happen before submitting this glslang PR
https://github.com/KhronosGroup/glslang/pull/2440. This commit will be
git-reverted after merging the glslang PR.

BUILD.gn

@@ -561,6 +561,7 @@ static_library("spvtools_opt") {
     "source/opt/eliminate_dead_functions_util.h",
     "source/opt/eliminate_dead_members_pass.cpp",
     "source/opt/eliminate_dead_members_pass.h",
+    "source/opt/empty_pass.h",
     "source/opt/feature_manager.cpp",
     "source/opt/feature_manager.h",
     "source/opt/fix_storage_class.cpp",

include/spirv-tools/optimizer.hpp

@@ -536,6 +536,18 @@ Optimizer::PassToken CreateDeadInsertElimPass();
 // eliminated with standard dead code elimination.
 Optimizer::PassToken CreateAggressiveDCEPass();
 
+// Creates an empty pass.
+// This is deprecated and will be removed.
+// TODO(jaebaek): remove this pass after handling glslang's broken unit tests.
+//                https://github.com/KhronosGroup/glslang/pull/2440
+Optimizer::PassToken CreatePropagateLineInfoPass();
+
+// Creates an empty pass.
+// This is deprecated and will be removed.
+// TODO(jaebaek): remove this pass after handling glslang's broken unit tests.
+//                https://github.com/KhronosGroup/glslang/pull/2440
+Optimizer::PassToken CreateRedundantLineInfoElimPass();
+
 // Creates a compact ids pass.
 // The pass remaps result ids to a compact and gapless range starting from %1.
 Optimizer::PassToken CreateCompactIdsPass();

source/opt/CMakeLists.txt

@@ -43,6 +43,7 @@ set(SPIRV_TOOLS_OPT_SOURCES
   eliminate_dead_functions_pass.h
   eliminate_dead_functions_util.h
   eliminate_dead_members_pass.h
+  empty_pass.h
   feature_manager.h
   fix_storage_class.h
   flatten_decoration_pass.h

source/opt/empty_pass.h

@@ -0,0 +1,36 @@
+// Copyright (c) 2020 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef SOURCE_OPT_EMPTY_PASS_H_
+#define SOURCE_OPT_EMPTY_PASS_H_
+
+#include "source/opt/pass.h"
+
+namespace spvtools {
+namespace opt {
+
+// Documented in optimizer.hpp
+class EmptyPass : public Pass {
+ public:
+  EmptyPass() {}
+
+  const char* name() const override { return "empty-pass"; }
+
+  Status Process() override { return Status::SuccessWithoutChange; }
+};
+
+}  // namespace opt
+}  // namespace spvtools
+
+#endif  // SOURCE_OPT_EMPTY_PASS_H_

source/opt/optimizer.cpp

@@ -753,6 +753,14 @@ Optimizer::PassToken CreateAggressiveDCEPass() {
       MakeUnique<opt::AggressiveDCEPass>());
 }
 
+Optimizer::PassToken CreatePropagateLineInfoPass() {
+  return MakeUnique<Optimizer::PassToken::Impl>(MakeUnique<opt::EmptyPass>());
+}
+
+Optimizer::PassToken CreateRedundantLineInfoElimPass() {
+  return MakeUnique<Optimizer::PassToken::Impl>(MakeUnique<opt::EmptyPass>());
+}
+
 Optimizer::PassToken CreateCompactIdsPass() {
   return MakeUnique<Optimizer::PassToken::Impl>(
       MakeUnique<opt::CompactIdsPass>());

source/opt/passes.h

@@ -35,6 +35,7 @@
 #include "source/opt/eliminate_dead_constant_pass.h"
 #include "source/opt/eliminate_dead_functions_pass.h"
 #include "source/opt/eliminate_dead_members_pass.h"
+#include "source/opt/empty_pass.h"
 #include "source/opt/fix_storage_class.h"
 #include "source/opt/flatten_decoration_pass.h"
 #include "source/opt/fold_spec_constant_op_and_composite_pass.h"