[PR47636] Fix tryEmitPrivate to handle non-constantarraytypes

As mentioned in the bug report, tryEmitPrivate chokes on the
MaterializeTemporaryExpr in the reproducers, since it assumes that if
there are elements, than it must be a ConstantArrayType. However, the
MaterializeTemporaryExpr (which matches exactly the AST when it is NOT a
global/static) has an incomplete array type.

This changes the section where the number-of-elements is non-zero to
properly handle non-CAT types by just extracting it as an array type
(since all we needed was the element type out of it).