[TCling][ROOT-9660] Do not fwd declare on library load enums Cling knows already

the fix consists in a rewrite of the parser of enum fwd declarations which come
from dictionaries. Those are created by Cling's forward declarator.
The previous implementation was assuming namespace declarations and enum fwd
declarations on different lines, e.g.

namespace mystuff{
enum  __attribute__((annotate("$clingAutoload$myenum.h"))) myenum : unsigned int;
}

while in reality the declaration looks like
namespace mystuff{enum  __attribute__((annotate("$clingAutoload$myenum.h"))) myenum : unsigned int;}