Fixes#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.
bc08e69959