Fix the swiftCxxStdlib link with explicit module build on Windows (#85904)

We currently support static linking swiftCxxStdlib only on Windows. When
C++ interop is enabled but swiftCxxStdlib isn't actually used or only
when its underlying std module is used, we incorrectly emitted the
dynamic version of the lib name and caused the link error. This change
fixes it by always adding it to the dependency.

Issue #85876

Author: hjyamauchi

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -620,6 +620,23 @@ ModuleDependencyScanner::getMainModuleDependencyInfo(ModuleDecl *mainModule) {
       break;
     }
 
+    if (ScanASTContext.LangOpts.EnableCXXInterop) {
+      StringRef mainModuleName = mainModule->getName().str();
+      if (mainModuleName != CXX_MODULE_NAME)
+        mainDependencies.addModuleImport(CXX_MODULE_NAME, /* isExported */ false,
+                                         AccessLevel::Public,
+                                         &alreadyAddedModules);
+      if (llvm::none_of(llvm::ArrayRef<StringRef>{CXX_MODULE_NAME,
+                            ScanASTContext.Id_CxxStdlib.str(), "std"},
+                        [mainModuleName](StringRef Name) {
+                          return mainModuleName == Name;
+                        }))
+        mainDependencies.addModuleImport(ScanASTContext.Id_CxxStdlib.str(),
+                                         /* isExported */ false,
+                                         AccessLevel::Public,
+                                         &alreadyAddedModules);
+    }
+
     // Add any implicit module names.
     for (const auto &import : importInfo.AdditionalUnloadedImports) {
       mainDependencies.addModuleImport(

test/ScanDependencies/win-explicit-cxx-interop-cxxstdlib.swift

@@ -0,0 +1,43 @@
+// REQUIRES: OS=windows-msvc
+
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -scan-dependencies -module-name NoCpp \
+// RUN:   -module-cache-path %t/clang-module-cache -disable-objc-interop \
+// RUN:   -cxx-interoperability-mode=default \
+// RUN:   %t/NoCpp.swift -o %t/NoCpp-deps.json -I %t 2>&1
+// RUN: %target-swift-frontend -scan-dependencies -module-name Cpp \
+// RUN:   -module-cache-path %t/clang-module-cache -disable-objc-interop \
+// RUN:   -cxx-interoperability-mode=default \
+// RUN:   %t/Cpp.swift -o %t/Cpp-deps.json -I %t 2>&1
+// RUN: %target-swift-frontend -scan-dependencies -module-name Crt \
+// RUN:   -module-cache-path %t/clang-module-cache -disable-objc-interop \
+// RUN:   -cxx-interoperability-mode=default \
+// RUN:   %t/Crt.swift -o %t/Crt-deps.json -I %t 2>&1
+// RUN: %FileCheck %s --check-prefix=NOCPP < %t/NoCpp-deps.json
+// RUN: %FileCheck %s --check-prefix=CPP < %t/Cpp-deps.json
+// RUN: %FileCheck %s --check-prefix=CRT < %t/Crt-deps.json
+
+// NOCPP:      "mainModuleName": "NoCpp"
+// NOCPP:      "linkName": "swiftCxxStdlib",
+// NOCPP-NEXT: "isStatic": true,
+
+// CPP:        "mainModuleName": "Cpp"
+// CPP:        "linkName": "swiftCxxStdlib",
+// CPP-NEXT:   "isStatic": true,
+
+// CRT:        "mainModuleName": "Crt"
+// CRT:        "linkName": "swiftCxxStdlib",
+// CRT-NEXT:   "isStatic": true,
+
+//--- NoCpp.swift
+// Empty
+
+//--- Cpp.swift
+import CxxStdlib
+
+//--- Crt.swift
+import CRT
+
+
+