🍒[cxx-interop] Do not expose enums with optional generic cases

Explanation:
The code path trying to generate the C++ header for enums that have
cases with associated values of a generic optional type resulted in null
pointer dereference. The proper fix is a bit more involved so this PR
just temporarily avoids exposing the problematic enums to C++.
Scope: C++ Reverse Interop
Risk: Low, we stop exposing enums that were crashing the compiler before.
Testing: Added a regression test.
Issue: rdar://129250756
Reviewer: @egorzhdan
Original PR: #74966

Author: Xazax-hun

lib/PrintAsClang/DeclAndTypePrinter.cpp

@@ -2910,6 +2910,9 @@ static bool isEnumExposableToCxx(const ValueDecl *VD,
       if (auto *params = elementDecl->getParameterList()) {
         for (const auto *param : *params) {
           auto paramType = param->getInterfaceType();
+          // TODO: properly support exporting these optionals.  
+          if (paramType->isOptional() && paramType->getOptionalObjectType()->isTypeParameter())
+            return false;
           if (DeclAndTypeClangFunctionPrinter::getTypeRepresentation(
                   printer.getTypeMapping(), printer.getInteropContext(),
                   printer, enumDecl->getModuleContext(), paramType)

test/Interop/SwiftToCxx/generics/generic-enum-in-cxx.swift

@@ -1,12 +1,12 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -typecheck -module-name Generics -clang-header-expose-decls=all-public -emit-clang-header-path %t/generics.h
+// RUN: %target-swift-frontend %s -typecheck -module-name Generics -enable-experimental-cxx-interop -emit-clang-header-path %t/generics.h
 // RUN: %FileCheck %s < %t/generics.h
-// RUN: %check-interop-cxx-header-in-clang(%t/generics.h -Wno-reserved-identifier)
+// RUN: %check-interop-cxx-header-in-clang(%t/generics.h -Wno-reserved-identifier -DSWIFT_CXX_INTEROP_HIDE_STL_OVERLAY)
 
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend %s -enable-library-evolution -typecheck -module-name Generics -clang-header-expose-decls=all-public -emit-clang-header-path %t/generics.h
+// RUN: %target-swift-frontend %s -enable-library-evolution -typecheck -module-name Generics -enable-experimental-cxx-interop -emit-clang-header-path %t/generics.h
 // RUN: %FileCheck %s < %t/generics.h
-// RUN: %check-interop-cxx-header-in-clang(%t/generics.h -Wno-reserved-identifier)
+// RUN: %check-interop-cxx-header-in-clang(%t/generics.h -Wno-reserved-identifier -DSWIFT_CXX_INTEROP_HIDE_STL_OVERLAY)
 
 // FIXME: remove the need for -Wno-reserved-identifier
 
@@ -89,6 +89,13 @@ public func inoutConcreteOpt(_ x: inout GenericOpt<UInt16>) {
     }
 }
 
+@frozen public enum GenericCustomType<T> {
+    case success(T?)
+    case failure
+}
+
+// CHECK: namespace Generics SWIFT_PRIVATE_ATTR SWIFT_SYMBOL_MODULE("Generics") {
+
 // CHECK: template<class T_0_0>
 // CHECK-NEXT: #ifdef __cpp_concepts
 // CHECK-NEXT: requires swift::isUsableInGenericContext<T_0_0>