[C++ interop] Fix reflection info compiler crash. (#26438)

When a c++ namespace IRGens for reflection, it triggers a path that
expects the namespace to have a particular form. Because it is always
empty, we can treat it just like an empty swift enum.

Author: pschuh

lib/IRGen/GenEnum.cpp

@@ -5900,6 +5900,13 @@ EnumImplStrategy::get(TypeConverter &TC, SILType type, EnumDecl *theEnum) {
                                          std::move(elementsWithNoPayload)));
   }
 
+  // namespace-like enums must be imported as empty decls.
+  if (theEnum->hasClangNode() && numElements == 0 && !theEnum->isObjC()) {
+    return std::unique_ptr<EnumImplStrategy>(new SingletonEnumImplStrategy(
+        TC.IGM, tik, alwaysFixedSize, numElements,
+        std::move(elementsWithPayload), std::move(elementsWithNoPayload)));
+  }
+
   // Enums imported from Clang or marked with @objc use C-compatible layout.
   if (theEnum->hasClangNode() || theEnum->isObjC()) {
     assert(elementsWithPayload.empty() && "C enum with payload?!");

lib/IRGen/GenReflection.cpp

@@ -607,7 +607,11 @@ class FieldTypeMetadataBuilder : public ReflectionMetadataBuilder {
   }
 
   void layout() override {
-    assert(!NTD->hasClangNode() || isa<StructDecl>(NTD));
+    if (NTD->hasClangNode()) {
+      auto *enumDecl = dyn_cast<EnumDecl>(NTD);
+      // Structs and namespace-like enums are ok.
+      assert(isa<StructDecl>(NTD) || (enumDecl && !enumDecl->hasCases()));
+    }
 
     PrettyStackTraceDecl DebugStack("emitting field type metadata", NTD);
     addNominalRef(NTD);

test/ClangImporter/cxx_interop_ir.swift

@@ -9,6 +9,12 @@ func indirectUsage() {
   useT(makeT())
 }
 
+// CHECK-LABEL: define hidden swiftcc %swift.type* @"$s6cxx_ir14reflectionInfo3argypXpSo2nsV1TV_tF"
+// CHECK: %0 = call swiftcc %swift.metadata_response @"$sSo2nsV1TVMa"({{i64|i32}} 0)
+func reflectionInfo(arg: namespacedT) -> Any.Type {
+  return type(of: arg)
+}
+
 // CHECK: define hidden swiftcc void @"$s6cxx_ir24namespaceManglesIntoName3argySo2nsV1TV_tF"
 func namespaceManglesIntoName(arg: namespacedT) {
 }