c++: Fix ICE with static_cast when converting from int[] [PR93862]

c++: Fix ICE with static_cast when converting from int[] [PR93862]

This ICEs since my patch for P0388, which allowed conversions to arrays
of unknown bound, but not the reverse, so these two static_casts are
ill-formed.

[expr.static.cast]/3 says that "cv1 T1" and "cv2 T2" have to be
reference-compatible and the comment in build_static_cast_1 says it too
but then we actually use reference_related_p...  Fixed thus.

2020-02-26  Marek Polacek  <polacek@redhat.com>

	PR c++/93862 - ICE with static_cast when converting from int[].
	* call.c (reference_compatible_p): No longer static.
	* cp-tree.h (reference_compatible_p): Declare.
	* typeck.c (build_static_cast_1): Use reference_compatible_p instead
	of reference_related_p.

	* g++.dg/cpp0x/rv-cast7.C: New test.