c++: pedwarn for empty unnamed enum in decl [PR67048]

c++: pedwarn for empty unnamed enum in decl [PR67048]

[dcl.dcl]/5 says that

  enum { };

is ill-formed, and since r197742 we issue a pedwarn.  However, the
pedwarn also fires for

   enum { } x;

which is well-formed.  So only warn when {} is followed by a ;.  This
should be correct since you can't have "enum {}, <whatever>" -- that
produces "expected unqualified-id before ',' token".

	PR c++/67048

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_enum_specifier): Warn about empty unnamed enum
	only when it's followed by a semicolon.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/enum42.C: New test.