Merge pull request #60757 from hyp/eng/expose-stdlib

[interop][SwiftToCxx] add a test to print out empty Swift stdlib module

Author: hyp

lib/PrintAsClang/ModuleContentsWriter.cpp

@@ -281,6 +281,10 @@ class ModuleWriter {
       forwardDeclare(ED);
     } else if (isa<AbstractTypeParamDecl>(TD)) {
       llvm_unreachable("should not see type params here");
+    } else if (isa<StructDecl>(TD) &&
+               TD->getModuleContext()->isStdlibModule()) {
+      // stdlib has some @_cdecl functions with structs.
+      return;
     } else {
       assert(false && "unknown local type decl");
     }

test/Interop/SwiftToCxx/stdlib/swift-stdlib-in-cxx.swift

@@ -0,0 +1,8 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -parse-as-library %platform-module-dir/Swift.swiftmodule/%module-target-triple.swiftinterface -enable-library-evolution -disable-objc-attr-requires-foundation-module -typecheck -module-name Swift -parse-stdlib -enable-experimental-cxx-interop -emit-clang-header-path %t/Swift.h  -experimental-skip-all-function-bodies
+// RUN: %FileCheck %s < %t/Swift.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/Swift.h -Wno-unused-private-field -Wno-unused-function)
+
+// CHECK: namespace Swift {
+// CHECK: } // namespace Swift