c++: new-expr clobber of constant-size array

c++: new-expr clobber of constant-size array

I previously tried to clobber an array as a whole, but fell back on a loop
due to issues with std::construct_at following the resolution of LWG3436.
But the loop seems to make life hard for the optimizers and it occurs to me
that for a one-element array we can just clobber the element type.

This also fixes some xfails in Warray-bounds-20.C.

gcc/cp/ChangeLog:

	* init.cc (build_new_1): Clobber a constant-bound array as a whole.

gcc/testsuite/ChangeLog:

	* g++.dg/warn/Warray-bounds-20.C: Remove xfails, add diags.