Fortran: fix TARGET attribute of associating entity in ASSOCIATE [PR112764]

The associating entity in an ASSOCIATE construct has the TARGET attribute
if and only if the selector is a variable and has either the TARGET or
POINTER attribute (e.g. F2018:11.1.3.3).

gcc/fortran/ChangeLog:

	PR fortran/112764
	* primary.cc (gfc_variable_attr): Set TARGET attribute of associating
	entity dependent on TARGET or POINTER attribute of selector.

gcc/testsuite/ChangeLog:

	PR fortran/112764
	* gfortran.dg/associate_62.f90: New test.

(cherry picked from commit 951a3e37)