[Frontend] Allow -typecheck-module-from-interface to emit no outputs

-typecheck-module-from-interface doesn't emit textual output as
configured in frontend option. The real output is diagnostics but that
is not tracked by output computer which is using the option.

Teach `swift-frontend` that it doesn't actually emit any real output and
tell serialization not to emit swiftmodule.

Author: cachemeifyoucan

lib/Frontend/FrontendOptions.cpp

@@ -769,14 +769,14 @@ bool FrontendOptions::doesActionProduceOutput(ActionType action) {
   case ActionType::EmitImportedModules:
   case ActionType::MergeModules:
   case ActionType::CompileModuleFromInterface:
-  case ActionType::TypecheckModuleFromInterface:
   case ActionType::DumpTypeInfo:
   case ActionType::EmitPCM:
   case ActionType::DumpPCM:
   case ActionType::ScanDependencies:
   case ActionType::PrintFeature:
     return true;
 
+  case ActionType::TypecheckModuleFromInterface:
   case ActionType::NoneAction:
   case ActionType::Immediate:
   case ActionType::REPL:
@@ -800,12 +800,12 @@ bool FrontendOptions::doesActionProduceTextualOutput(ActionType action) {
   case ActionType::Immediate:
   case ActionType::REPL:
   case ActionType::EmitPCM:
+  case ActionType::TypecheckModuleFromInterface:
     return false;
 
   case ActionType::Parse:
   case ActionType::ResolveImports:
   case ActionType::Typecheck:
-  case ActionType::TypecheckModuleFromInterface:
   case ActionType::DumpParse:
   case ActionType::DumpInterfaceHash:
   case ActionType::DumpAST:

lib/Frontend/Serialization.cpp

@@ -65,8 +65,10 @@ void swift::serializeToBuffers(
     std::unique_ptr<llvm::MemoryBuffer> *moduleDocBuffer,
     std::unique_ptr<llvm::MemoryBuffer> *moduleSourceInfoBuffer,
     const SILModule *M) {
+  // Serialization output is disabled.
+  if (options.OutputPath.empty())
+    return;
 
-  assert(!options.OutputPath.empty());
   {
     FrontendStatsTracer tracer(getContext(DC).Stats,
                                "Serialization, swiftmodule, to buffer");

test/CAS/cas-explicit-module-map.swift

@@ -120,11 +120,11 @@
 // RUN: %target-swift-frontend -typecheck-module-from-interface %t/Foo.swiftinterface -disable-implicit-swift-modules \
 // RUN:   -module-cache-path %t.module-cache -explicit-swift-module-map-file @%t/map.casid  \
 // RUN:   -cache-compile-job -cas-path %t/cas -allow-unstable-cache-key-for-testing -swift-version 5 -enable-library-evolution \
-// RUN:   -explicit-interface-module-build -o /dev/null -Rcache-compile-job 2>&1 | %FileCheck %s --check-prefix=VERIFY-OUTPUT --check-prefix=CACHE-MISS
+// RUN:   -explicit-interface-module-build -Rcache-compile-job 2>&1 | %FileCheck %s --check-prefix=VERIFY-OUTPUT --check-prefix=CACHE-MISS
 // RUN: %target-swift-frontend -typecheck-module-from-interface %t/Foo.swiftinterface -disable-implicit-swift-modules \
 // RUN:   -module-cache-path %t.module-cache -explicit-swift-module-map-file @%t/map.casid  \
 // RUN:   -cache-compile-job -cas-path %t/cas -allow-unstable-cache-key-for-testing -swift-version 5 -enable-library-evolution \
-// RUN:   -explicit-interface-module-build -o %t/check.swiftmodule -Rcache-compile-job 2>&1 | %FileCheck %s --check-prefix=VERIFY-OUTPUT --check-prefix=CACHE-HIT
+// RUN:   -explicit-interface-module-build -Rcache-compile-job 2>&1 | %FileCheck %s --check-prefix=VERIFY-OUTPUT --check-prefix=CACHE-HIT
 
 // CHECK-DAG: loaded module 'A'
 // CHECK-DAG: loaded module 'B'

test/ScanDependencies/explicit-module-map-typecheck-module-from-interface.swift

@@ -35,7 +35,7 @@
 // RUN: echo "}]" >> %/t/inputs/map.json
 
 // RUN: %target-swift-frontend -typecheck-module-from-interface %t/Foo.swiftinterface -module-cache-path %t.module-cache \
-// RUN:   -o /dev/null -explicit-interface-module-build -explicit-swift-module-map-file %t/inputs/map.json -Rmodule-loading -Xcc -Rmodule-import 2>&1 | %FileCheck %s
+// RUN:   -explicit-interface-module-build -explicit-swift-module-map-file %t/inputs/map.json -Rmodule-loading -Xcc -Rmodule-import 2>&1 | %FileCheck %s
 
 // CHECK-DAG: loaded module 'Swift'
 // CHECK-DAG: loaded module '_StringProcessing'