PR c++/96442: Improved error recovery in enumerations.

PR c++/96442: Improved error recovery in enumerations.

This patch is a revised fix for PR c++/96442 providing a cleaner
solution, setting ENUM_UNDERLYING_TYPE to integer_type_node when
issuing an error, so that this invariant holds during the parser's
error recovery.

2022-06-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/cp/ChangeLog
	PR c++/96442
	* decl.cc (start_enum): When emitting a "must be integral" error,
	set ENUM_UNDERLYING_TYPE to integer_type_node, to avoid an ICE
	downstream in build_enumeration.

gcc/testsuite/ChangeLog
	PR c++/96442
	* g++.dg/parse/pr96442.C: New test case.