c++: simplify norm_cache manipulation

c++: simplify norm_cache manipulation

We can avoid performing two norm_cache lookups during normalization of a
concept-id by allocating and inserting a norm_entry* before rather than
after the fact, which is simpler and cheaper.

gcc/cp/ChangeLog:

	* constraint.cc (normalize_concept_check): Avoid having to do
	two norm_cache lookups.  Remove unnecessary early exit for an
	ill-formed concept definition.