OpenMP/Fortran: Permit impure ELEMENTAL in omp directives

OpenMP/Fortran: Permit impure ELEMENTAL in omp directives

OpenMP since 4.5 permits IMPURE ELEMENTAL in directives and
the code already only checked for PURE.

gcc/fortran/ChangeLog:

	* parse.c (decode_omp_directive): Remove "or ELEMENTAL"
	from "in PURE" error message.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/pr79154-1.f90: Update dg-*;
	add an impure elemental example.
	* gfortran.dg/gomp/pr79154-2.f90: Likewise.