[interop][SwiftToCxx] NFC, add test to cover cross-module enum refs

Author: hyp

test/Interop/SwiftToCxx/cross-module-refs/Inputs/enums.swift

@@ -0,0 +1,9 @@
+// Large enum passed indirectly.
+public enum LargeEnum {
+    case A(x1: Int64, x2: Int64, x3: Int64, x4: Int64, x5: Int64)
+    case B
+}
+
+public func passThroughLargeEnum(_ x: LargeEnum) -> LargeEnum {
+    return x
+}

test/Interop/SwiftToCxx/cross-module-refs/imported-enum-refs-in-cxx.swift

@@ -0,0 +1,42 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend %S/Inputs/enums.swift -module-name Enums -emit-module -emit-module-path %t/Enums.swiftmodule -clang-header-expose-public-decls -emit-clang-header-path %t/enums.h
+
+// RUN: %target-swift-frontend %s -typecheck -module-name UsesEnums -I %t -clang-header-expose-public-decls -emit-clang-header-path %t/uses-enums.h
+
+// FIXME: add import automatically?
+// RUN: echo '#include "enums.h"' > %t/fixed-uses-enums.h
+// RUN: cat %t/uses-enums.h     >> %t/fixed-uses-enums.h
+
+// RUN: %FileCheck %s < %t/uses-enums.h
+
+// RUN: %check-interop-cxx-header-in-clang(%t/fixed-uses-enums.h)
+
+import Enums
+
+public struct UsesEnumsLargeEnum {
+    public func passThroughStructSeveralI64(_ y: LargeEnum) -> LargeEnum {
+        return y
+    }
+
+    public let x: LargeEnum
+}
+
+// CHECK: inline Enums::LargeEnum UsesEnumsLargeEnum::passThroughStructSeveralI64(const Enums::LargeEnum& y) const {
+// CHECK-NEXT: return Enums::_impl::_impl_LargeEnum::returnNewValue([&](void * _Nonnull result) {
+// CHECK-NEXT:   _impl::$s9UsesEnums0aB9LargeEnumV27passThroughStructSeveralI64y0B00cD0OAGF(result, Enums::_impl::_impl_LargeEnum::getOpaquePointer(y), _getOpaquePointer());
+// CHECK-NEXT: });
+// CHECK-NEXT: }
+// CHECK-NEXT: inline Enums::LargeEnum UsesEnumsLargeEnum::getX() const {
+// CHECK-NEXT: return Enums::_impl::_impl_LargeEnum::returnNewValue([&](void * _Nonnull result) {
+// CHECK-NEXT:   _impl::$s9UsesEnums0aB9LargeEnumV1x0B00cD0Ovg(result, _getOpaquePointer());
+// CHECK-NEXT: });
+// CHECK-NEXT: }
+
+
+public func inoutLargeEnum(_ s: inout LargeEnum) {
+    return s = LargeEnum.B
+}
+
+// CHECK: inline void inoutLargeEnum(Enums::LargeEnum& s) noexcept {
+// CHECK-NEXT: return _impl::$s9UsesEnums14inoutLargeEnumyy0B00dE0OzF(Enums::_impl::_impl_LargeEnum::getOpaquePointer(s));
+// CHECK-NEXT: }