[6.2][cxx-interop] Fix duplicate symbol error with default arguments

Explanation: We synthesize a Swift function that only calls a C++ funtion that produces the default argument. We produce this function for each module that imports the C++ function with the default argument. This symbol needs to be public as it is created in the context of the Clang module and used from the Swift module. To avoid the linker errors, we always emit this function into the client which is also the right thing to do as the whole body is a single function call.
Issues: rdar://160182651
Original PRs: swiftlang#82333
Risk: Low, this is merged to main in early July and no one complained
yet. And the change is straightforward.
Testing: Added a compiler test.
Reviewers: @egorzhdan

Author: Xazax-hun

lib/ClangImporter/SwiftDeclSynthesizer.cpp

@@ -12,6 +12,7 @@
 
 #include "SwiftDeclSynthesizer.h"
 #include "swift/AST/ASTMangler.h"
+#include "swift/AST/Attr.h"
 #include "swift/AST/AttrKind.h"
 #include "swift/AST/Builtins.h"
 #include "swift/AST/Decl.h"
@@ -2522,6 +2523,8 @@ SwiftDeclSynthesizer::makeDefaultArgument(const clang::ParmVarDecl *param,
       ImporterImpl.ImportedHeaderUnit);
   funcDecl->setBodySynthesizer(synthesizeDefaultArgumentBody, (void *)param);
   funcDecl->setAccess(AccessLevel::Public);
+  funcDecl->getAttrs().add(new (ctx)
+                               AlwaysEmitIntoClientAttr(/*IsImplicit=*/true));
 
   ImporterImpl.defaultArgGenerators[param] = funcDecl;
 

test/Interop/Cxx/function/default-arguments-multifile.swift

@@ -0,0 +1,46 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: mkdir -p %t/artifacts
+
+// Multiple usages in the same module.
+// RUN: %target-build-swift %t/main.swift %t/b.swift %t/c.swift -cxx-interoperability-mode=upcoming-swift -I %S/Inputs -o %t/artifacts/out
+// RUN: %empty-directory(%t/artifacts)
+
+// Multiple usages across different modules.
+// RUN: %target-build-swift -emit-library -module-name BarLibrary -emit-module -emit-module-path %t/artifacts/BarLibrary.swiftmodule %t/b.swift %t/c.swift -cxx-interoperability-mode=upcoming-swift -I %S/Inputs -o %t/artifacts/%target-library-name(BarLibrary)
+// RUN: %target-build-swift %t/uses-library.swift -cxx-interoperability-mode=upcoming-swift -I %S/Inputs -I %t/artifacts -L %t/artifacts -lBarLibrary -o %t/artifacts/uses-library
+
+// FIXME: Windows test can be enabled once merge-modules step is removed from the old driver,
+// or the Windows CI starts to use the new driver to compile the compiler.
+// The windows toolchains still use the new driver so the CI failure worked
+// around here should not affect users on Windows.
+// UNSUPPORTED: OS=windows-msvc
+
+//--- main.swift
+import DefaultArguments
+public func foo() {
+  let _ = isZero()
+}
+foo()
+bar()
+baz()
+
+//--- b.swift
+import DefaultArguments
+public func bar() {
+  let _ = isZero()
+}
+
+//--- c.swift
+import DefaultArguments
+public func baz() {
+  let _ = isZero(123)
+}
+
+//--- uses-library.swift
+import DefaultArguments
+import BarLibrary
+
+let _ = isZero()
+bar()
+baz()

test/Interop/Cxx/function/lit.local.cfg

@@ -0,0 +1,6 @@
+# Make a local copy of the environment.
+config.environment = dict(config.environment)
+
+# FIXME: deserialization failure with the old driver in default-arguments-multifile.swift
+if 'SWIFT_USE_OLD_DRIVER' in config.environment: del config.environment['SWIFT_USE_OLD_DRIVER']
+if 'SWIFT_AVOID_WARNING_USING_OLD_DRIVER' in config.environment: del config.environment['SWIFT_AVOID_WARNING_USING_OLD_DRIVER']