Avoid false FileDependnecies among Implicitly-build Swift and Clang modules

`InterfaceSubContextDelegateImpl` causes sub-instances to inherit `-fmodule-map-file=` options.
Those Module Maps become file dependencies of all downstream PCMs and their depending Swift modules, even though they really aren't.
This causes frequent re-builds of the Module Cache contents when seemingly-unrelated files are touched.

Explicit Module Builds rely on these options for building Swift Interface files, so for now we just disable inheritance of these options in Implicit Module builds.

Author: artemcm

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1292,18 +1292,28 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
   // required by sourcekitd.
   subClangImporterOpts.DetailedPreprocessingRecord =
     clangImporterOpts.DetailedPreprocessingRecord;
+
   // We need to add these extra clang flags because explict module building
   // related flags are all there: -fno-implicit-modules, -fmodule-map-file=,
   // and -fmodule-file=.
   // If we don't add these flags, the interface will be built with implicit
   // PCMs.
-  subClangImporterOpts.ExtraArgs = clangImporterOpts.ExtraArgs;
-  for (auto arg: subClangImporterOpts.ExtraArgs) {
-    GenericArgs.push_back("-Xcc");
-    GenericArgs.push_back(ArgSaver.save(arg));
+  // FIXME: With Implicit Module Builds, if sub-invocations inherit `-fmodule-map-file=` options,
+  // those modulemaps become File dependencies of all downstream PCMs and their depending Swift
+  // modules, triggering unnecessary re-builds. We work around this by only inheriting these options
+  // when building with explicit modules. While this problem will not manifest with Explicit Modules
+  // (which do not use the ClangImporter to build PCMs), we may still need a better way to
+  // decide which options must be inherited here.
+  if (LoaderOpts.disableImplicitSwiftModule) {
+    subClangImporterOpts.ExtraArgs = clangImporterOpts.ExtraArgs;
+    for (auto arg : subClangImporterOpts.ExtraArgs) {
+      GenericArgs.push_back("-Xcc");
+      GenericArgs.push_back(ArgSaver.save(arg));
+    }
   }
 
-  // Tell the genericSubInvocation to serialize dependency hashes if asked to do so.
+  // Tell the genericSubInvocation to serialize dependency hashes if asked to do
+  // so.
   auto &frontendOpts = genericSubInvocation.getFrontendOptions();
   frontendOpts.SerializeModuleInterfaceDependencyHashes =
     serializeDependencyHashes;

test/ModuleInterface/Inputs/implicit-options-inheritance/CIMod.h

@@ -0,0 +1 @@
+void funcCI(void);

test/ModuleInterface/Inputs/implicit-options-inheritance/SIMod.swiftinterface

@@ -0,0 +1,4 @@
+// swift-interface-format-version: 1.0
+// swift-module-flags: -swift-version 5 -enforce-exclusivity=checked -module-name SIMod 
+import CIMod
+

test/ModuleInterface/Inputs/implicit-options-inheritance/module.modulemap

@@ -0,0 +1,4 @@
+module CIMod {
+  header "CIMod.h"
+  export *
+}

test/ModuleInterface/Inputs/implicit-options-inheritance/test-dummy.modulemap

@@ -0,0 +1,4 @@
+module Dummy {
+  header "Dummy.h"
+  export *
+}

test/ModuleInterface/no-implicit-extra-clang-opts.swift

@@ -0,0 +1,25 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/ModuleCache)
+
+import SIMod
+
+// Step 0: Copy relevant files into the temp dir which will serve as the search path
+// RUN: cp %S/Inputs/implicit-options-inheritance/module.modulemap %t/module.modulemap
+// RUN: cp %S/Inputs/implicit-options-inheritance/CIMod.h %t/CIMod.h
+// RUN: cp %S/Inputs/implicit-options-inheritance/SIMod.swiftinterface %t/SIMod.swiftinterface
+
+// Step 1: Build this file, causing an implicit build of SIMod and CIMod into the module cache.
+// Pass in a clang arg pointing it to a modulemap that has nothing to do with downstream modules and is not on the search path.
+
+// RUN: %target-swift-frontend -emit-module -module-name no-implicit-extra-clang-maps -o %t/no-implicit-extra-clang-maps.swiftmodule %s -I %t -Xcc -fmodule-map-file=%S/Inputs/implicit-options-inheritance/test-dummy.modulemap -module-cache-path %t/ModuleCache
+
+// Step 2: Touch the dummy modulemap we passed in with `-Xcc -fmodule-map-file` above.
+// RUN: touch %S/Inputs/implicit-options-inheritance/test-dummy.modulemap
+
+// Step 3: Re-build this file, and ensure we are not re-building SIMod due to a dependency on the dummy file
+// RUN: %target-swift-frontend -emit-module -module-name no-implicit-extra-clang-maps -o %t/no-implicit-extra-clang-maps.swiftmodule %s -I %t -Xcc -fmodule-map-file=%S/Inputs/implicit-options-inheritance/test-dummy.modulemap -module-cache-path %t/ModuleCache -Rmodule-interface-rebuild 2>&1 | %FileCheck -allow-empty %s
+
+// Step 4: Ensure that SIMod was not re-built
+// CHECK-NOT: remark: rebuilding module 'SIMod' from interface
+// CHECK-NOT: note: cached module is out of date
+// CHECK-NOT: note: dependency is out of date