[clang][modules] Fix crash in enum visibility lookup for C++20 header units (llvm#166272)

Fixes llvm#165445.

Fixes a crash when `ASTWriter::GenerateNameLookupTable` processes enum
constants from C++20 header units.

The special handling for enum constants, introduced in fccc6ee, doesn't
account for declarations whose owning module is a C++20 header unit. It
calls `isNamedModule()` on the result of
`getTopLevelOwningNamedModule()`, which returns null for header units,
causing a null pointer dereference.

(cherry picked from commit bc08e69)

Author: naveen-seth

clang/lib/Serialization/ASTWriter.cpp

@@ -4340,8 +4340,7 @@ class ASTDeclContextNameLookupTrait
     // parent of parent. We DON'T remove the enum constant from its parent. So
     // we don't need to care about merging problems here.
     if (auto *ECD = dyn_cast<EnumConstantDecl>(D);
-        ECD && DC.isFileContext() && ECD->getOwningModule() &&
-        ECD->getTopLevelOwningNamedModule()->isNamedModule()) {
+        ECD && DC.isFileContext() && ECD->getTopLevelOwningNamedModule()) {
       if (llvm::all_of(
               DC.noload_lookup(
                   cast<EnumDecl>(ECD->getDeclContext())->getDeclName()),

clang/test/Modules/crash-enum-visibility-with-header-unit.cppm

@@ -0,0 +1,46 @@
+// Fixes #165445
+
+// RUN: rm -rf %t
+// RUN: mkdir -p %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 -x c++-user-header %t/header.h \
+// RUN:   -emit-header-unit -o %t/header.pcm
+//
+// RUN: %clang_cc1 -std=c++20 %t/A.cppm -fmodule-file=%t/header.pcm \
+// RUN:   -emit-module-interface -o %t/A.pcm
+// 
+// RUN: %clang_cc1 -std=c++20 %t/B.cppm -fmodule-file=%t/header.pcm \
+// RUN:   -emit-module-interface -o %t/B.pcm
+//
+// RUN: %clang_cc1 -std=c++20 %t/use.cpp \
+// RUN:   -fmodule-file=A=%t/A.pcm -fmodule-file=B=%t/B.pcm  \
+// RUN:   -fmodule-file=%t/header.pcm \
+// RUN:   -verify -fsyntax-only
+
+//--- enum.h
+enum E { Value };
+
+//--- header.h
+#include "enum.h"
+
+//--- A.cppm
+module;
+#include "enum.h"
+export module A;
+
+auto e = Value;
+
+//--- B.cppm
+export module B;
+import "header.h";
+
+auto e = Value;
+
+//--- use.cpp
+// expected-no-diagnostics
+import A;
+import B;
+#include "enum.h"
+
+auto e = Value;