PR20346: fix aggregate initialization / template instantiation bug:

If, during the initial parse of a template, we perform aggregate initialization
and form an implicit value initialization for an array type, then when we come
to instantiate the template and redo the initialization step, we would try to
match the implicit value initialization up against an array *element*, not to
the complete array.

Remarkably, we've had this bug since ~the dawn of time, but only noticed it
recently.