c++: partially initialized constexpr array [PR99700]

c++: partially initialized constexpr array [PR99700]

Here, reduced_constant_expression_p is incorrectly returning true for a
partially initialized array CONSTRUCTOR (in C++20 mode) because when the
CONSTRUCTOR_NO_CLEARING flag is set, the predicate doesn't check that
the CONSTRUCTOR spans the entire array like it does for class CONSTRUCTORS.
This patch adds a dedicated loop for the array case that simultaneously
verifies the CONSTRUCTOR spans the entire array and is made up of valid
constant expressions.

gcc/cp/ChangeLog:

	PR c++/99700
	* constexpr.c (reduced_constant_expression_p): For array
	CONSTRUCTORs, use a dedicated loop that additionally verifies
	the CONSTRUCTOR spans the entire array.

gcc/testsuite/ChangeLog:

	PR c++/99700
	* g++.dg/cpp2a/constexpr-init21.C: New test.