PR c++/95999: Improved error recovery in enumeration lists.

This patch resolves PR c++/95999 which is an ICE-after-error regression
in the g++ front-end.  When parsing an enumerator list, the C++ parser
assumes that cp_parser_constant_expression always returns either an
INTEGER_CST or error_mark_node, but in the testcase reported in the
PR, it actually returns a VAR_DECL.

The usual (but perhaps controversial) design philosophy is that the
routine that reports the error normally has a duty to indicate this to
the rest of the compiler (via error_mark_node), but here the return
value from calling require_rvalue_constant_expression (parser.cc:10666)
is ignored.  I initially experimented with setting EXPRESSION to
error_mark_node here in cp_parser_constant_expression but (perhaps
conveniently) that's insufficient to resolve the problem.  The simple
fix in this patch is to tweak the two places that require INTEGER_CST
to treat all other tree types as though they are error_mark_node.

2022-03-10  Roger Sayle  <roger@nextmovesoftware.com>

gcc/cp/ChangeLog
	PR c++/95999
	* decl.cc (finish_enum_value_list): If VALUE isn't an INTEGER_CST
	consider it to be zero (i.e. treat it like error_mark_node).
	(build_enumerator): Likewise, if PREV_VALUE isn't an INTEGER_CST,
	set VALUE to error_mark_node.

gcc/testsuite/ChangeLog
	PR c++/95999
	* g++.dg/parse/pr95999.C: New test case.