[interop][SwiftToCxx] add a test that verifies that we can emit C++ header from previously built swiftmodule

Author: hyp

test/Interop/SwiftToCxx/core/gen-header-for-module.swift

@@ -0,0 +1,17 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %s -emit-module -module-name Core -o %t
+// RUN: %target-swift-frontend -parse-as-library %t/Core.swiftmodule -typecheck -module-name Core -clang-header-expose-public-decls -emit-clang-header-path %t/core.h
+// RUN: %FileCheck %s < %t/core.h
+
+// RUN: %empty-directory(%t-evo)
+// RUN: %target-swift-frontend %s -emit-module -enable-library-evolution -module-name Core -o %t-evo
+// RUN: %target-swift-frontend -parse-as-library %t-evo/Core.swiftmodule -enable-library-evolution -typecheck -module-name Core -clang-header-expose-public-decls -emit-clang-header-path %t-evo/core.h
+// RUN: %FileCheck %s < %t-evo/core.h
+
+
+public func reprintedInImportedModule() -> Int {
+    return 42
+}
+
+// CHECK: namespace Core {
+// CHECK: swift::Int reprintedInImportedModule() noexcept SWIFT_WARN_UNUSED_RESULT {