c++: Don't allow defining types in enum-base [PR96380]

In r11-2064 I made cp_parser_enum_specifier commit to tentative parse
when seeing a '{'.  That still looks like the correct thing to do, but
it caused an ICE-on-invalid as well as accepts-invalid.

When we have something sneaky like this, which is broken in multiple
ways:

  template <class>
  enum struct c : union enum struct c { e = b, f = a };

we parse the "enum struct c" part (that's OK) and then we see that
we have an enum-base, so we consume ':' and then parse the type-specifier
that follows the :.  "union enum" is clearly invalid, but we're still
parsing tentatively and we parse everything up to the ;, and then
throw away the underlying type.  We parsed everything because we were
tricked into parsing an enum-specifier in an enum-base of another
enum-specifier!  Not good.

Since the grammar for enum-base doesn't allow a defining-type-specifier,
only a type-specifier, we should set type_definition_forbidden_message
which fixes all the problems in this PR.

gcc/cp/ChangeLog:

	PR c++/96380
	* parser.c (cp_parser_enum_specifier): Don't allow defining
	types in enum-base.

gcc/testsuite/ChangeLog:

	PR c++/96380
	* g++.dg/cpp0x/enum_base4.C: New test.
	* g++.dg/cpp0x/enum_base5.C: New test.

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