Fortran/OpenMP: Warn when mapping polymorphic variables

Fortran/OpenMP: Warn when mapping polymorphic variables

OpenMP (TR13) states for Fortran:
* For map: "If a list item has polymorphic type, the behavior is unspecified."
* "If the firstprivate clause is on a target construct and a variable is of
  polymorphic type, the behavior is unspecified."
which this commit now warns for.

gcc/fortran/ChangeLog:

	* openmp.cc (resolve_omp_clauses): Diagnose polymorphic mapping.
	* trans-openmp.cc (gfc_omp_finish_clause): Warn when
	polymorphic variable is implicitly mapped.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/polymorphic-mapping.f90: New test.
	* gfortran.dg/gomp/polymorphic-mapping-2.f90: New test.