ASTWriter: Tighten up assertions when emitting a declaration name.

clang has recently started to warn about the enum compares:
lib/Serialization/ASTWriter.cpp:2760:31: warning: comparison of literal 256 with expression of type
'clang::DeclarationName::NameKind' is always true [-Wtautological-constant-out-of-range-compare]

llvm-svn: 164220