[clang][Modules] Respect -fno-cxx-modules as a driver flag (llvm#150349)

The mentioned flag is already both a cc1 & driver flag; however, whether
it is respected was tied to either:
1. Whether it was passed as a cc1 option (`Xclang`)
2. or `-fmodules` accompanying it

This poses a consistency problem where `std=c++20` enables the modules
feature, independent of other module settings.
This patch resolves this issue by checking for the presence
unconditionally & passing it down to cc1 when applicable.

Author: cyndyishida

clang/lib/Driver/ToolChains/Clang.cpp

@@ -3881,17 +3881,17 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
                                  const ArgList &Args, const InputInfo &Input,
                                  const InputInfo &Output, bool HaveStd20,
                                  ArgStringList &CmdArgs) {
-  bool IsCXX = types::isCXX(Input.getType());
-  bool HaveStdCXXModules = IsCXX && HaveStd20;
+  const bool IsCXX = types::isCXX(Input.getType());
+  const bool HaveStdCXXModules = IsCXX && HaveStd20;
   bool HaveModules = HaveStdCXXModules;
 
   // -fmodules enables the use of precompiled modules (off by default).
   // Users can pass -fno-cxx-modules to turn off modules support for
   // C++/Objective-C++ programs.
+  const bool AllowedInCXX = Args.hasFlag(options::OPT_fcxx_modules,
+                                         options::OPT_fno_cxx_modules, true);
   bool HaveClangModules = false;
   if (Args.hasFlag(options::OPT_fmodules, options::OPT_fno_modules, false)) {
-    bool AllowedInCXX = Args.hasFlag(options::OPT_fcxx_modules,
-                                     options::OPT_fno_cxx_modules, true);
     if (AllowedInCXX || !IsCXX) {
       CmdArgs.push_back("-fmodules");
       HaveClangModules = true;
@@ -3900,6 +3900,9 @@ static bool RenderModulesOptions(Compilation &C, const Driver &D,
 
   HaveModules |= HaveClangModules;
 
+  if (HaveModules && !AllowedInCXX)
+    CmdArgs.push_back("-fno-cxx-modules");
+
   // -fmodule-maps enables implicit reading of module map files. By default,
   // this is enabled if we are using Clang's flavor of precompiled modules.
   if (Args.hasFlag(options::OPT_fimplicit_module_maps,

clang/test/Driver/modules.mm

@@ -3,6 +3,9 @@
 // RUN: %clang -fmodules -fno-cxx-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-MODULES %s
 // CHECK-NO-MODULES-NOT: -fmodules
 
+// RUN: %clang -std=c++20 -fno-cxx-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-NO-CPP-20-MODULES %s
+// CHECK-NO-CPP-20-MODULES: -fno-cxx-modules
+
 // RUN: %clang -fmodules -### %s 2>&1 | FileCheck -check-prefix=CHECK-HAS-MODULES %s
 // RUN: %clang -fmodules -fno-cxx-modules -fcxx-modules -### %s 2>&1 | FileCheck -check-prefix=CHECK-HAS-MODULES %s
 // CHECK-HAS-MODULES: -fmodules