If build_static_cast_1 returns false, and one or both type is a
pointer/reference to an incomplete class, print a note saying so. This
doesn't attempt to check whether the static_cast failed because the
type is incomplete (which was checked inside build_static_cast_1 but
not in the caller). It just informs the user that it is incomplete,
which is probably the most likely reason.
PR c++/52321
* typeck.c (build_static_cast): Print a note when the destination
type or the operand is a pointer/reference to incomplete class type.
Tested powerpc64le-linux. OK for trunk?