[AST][NFC] Remove a superfluous enum in ObjCMethodDeclBitfields added in r338641

I originally added this enum to avoid including Basic/IdentifierTable.h
in AST/DeclBase.h. However I did not realise it is already included
transitively by AST/DeclarationName.h. Therefore remove this enum and
explicitly include Basic/IdentifierTable.h

llvm-svn: 342731