OpenACC: fix privatization of by-reference arrays

OpenACC: fix privatization of by-reference arrays

Replacing of a by-reference variable in a private clause by a local variable
makes sense; however, for arrays, the size is not directly known by the type.
This causes an ICE via create_tmp_var which indirectly invokes
force_constant_size in this case - but the latter only handled Ada.

gcc/ChangeLog:

        * gimplify.c (localize_reductions): Do not create local
	variable for privatized arrays.

libgomp/ChangeLog:

        * testsuite/libgomp.oacc-fortran/privatized-ref-2.f90: New test.