[Implicit Module Builds] Do not query CxxStdlib Swift overlay for textual modules which were not built with c++interop

When the compiler is building a module without a defined formal C++ interop mode (e.g. building a textual interface which specifies it was built without C++ interop enabled), avoid looking up the C++ standard library Swift overlay for it. This is required for the case of the Darwin module, for example, which includes headers which map to C++ stdlib headers when the compiler is operating in C++ interop mode, but the C++ standard library Swift overlay module itself depends on 'Darwin', which results in a cycle. To resolve such situations, we can rely on the fact that Swift textual interfaces of modules which were not built with C++ interop must be able to build without importing the C++ standard library Swift overlay, so we avoid specifying it as a dependency for such modules. The primary source module, as well as Swift textual module dependencies which were built with C++ interop will continue getting a direct depedency of the 'CxxStdlib' Swift module.

This was previously fixed in the dependency scanner for explicitly-built modules in #81415.

Author: artemcm

lib/ClangImporter/ClangImporter.cpp

@@ -4388,7 +4388,14 @@ ModuleDecl *ClangModuleUnit::getOverlayModule() const {
     // FIXME: Include proper source location.
     ModuleDecl *M = getParentModule();
     ASTContext &Ctx = M->getASTContext();
-    auto overlay = Ctx.getOverlayModule(this);
+
+    ModuleDecl *overlay = nullptr;
+    // During compilation of a textual interface with no formal C++ interop mode,
+    // i.e. it was built without C++ interop, avoid querying the 'CxxStdlib' overlay
+    // for it, since said overlay was not used during compilation of this module.
+    if (!importer::isCxxStdModule(clangModule) || Ctx.LangOpts.FormalCxxInteropMode)
+      overlay = Ctx.getOverlayModule(this);
+
     if (overlay) {
       Ctx.addLoadedModule(overlay);
     } else {
@@ -4408,7 +4415,8 @@ ModuleDecl *ClangModuleUnit::getOverlayModule() const {
     // If this Clang module is a part of the C++ stdlib, and we haven't loaded
     // the overlay for it so far, it is a split libc++ module (e.g. std_vector).
     // Load the CxxStdlib overlay explicitly.
-    if (!overlay && importer::isCxxStdModule(clangModule)) {
+    if (!overlay && importer::isCxxStdModule(clangModule) &&
+        Ctx.LangOpts.FormalCxxInteropMode) {
       ImportPath::Module::Builder builder(Ctx.Id_CxxStdlib);
       overlay = owner.loadModule(SourceLoc(), std::move(builder).get());
     }