c++: Fix tentative parsing of enum-specifier [PR96077]

c++: Fix tentative parsing of enum-specifier [PR96077]

Here's an interesting issue: in this code a ) is missing:

  enum { E = (2 } e;

but we compile the code anyway, and E is set to 0 in build_enumerator,
which is sneaky.

The problem is that cp_parser_enum_specifier parses tentatively, because
when we see the enum keyword, we don't know yet if we'll find an
enum-specifier, opaque-enum-declaration, or elaborated-enum-specifier.

In this test when we call cp_parser_enumerator_list we're still parsing
tentatively, and as a consequence, parens.require_close (parser) in
cp_parser_primary_expression doesn't report any errors.  But we only go
on to parse the enumerator-list after we've seen a {, at which point we
might as well commit -- we know we're dealing with an enum-specifier.

gcc/cp/ChangeLog:

	PR c++/96077
	* parser.c (cp_parser_enum_specifier): Commit to tentative parse
	after we've seen an opening brace.

gcc/testsuite/ChangeLog:

	PR c++/96077
	* g++.dg/parse/enum14.C: New test.