OpenMP/Fortran: Permit pure directives inside PURE

Update permitted directives for directives marked in OpenMP's 5.2 as pure.
To ensure that list is updated, unimplemented directives are placed into
pure-2.f90 such the test FAILs once a known to be pure directive is
implemented without handling its pureness.

gcc/fortran/ChangeLog:

	* parse.cc (decode_omp_directive): Accept all pure directives
	inside a PURE procedures; handle 'error at(execution).

libgomp/ChangeLog:

	* libgomp.texi (OpenMP 5.2): Mark pure-directive handling as 'Y'.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/nothing-2.f90: Remove one dg-error.
	* gfortran.dg/gomp/pr79154-2.f90: Update expected dg-error wording.
	* gfortran.dg/gomp/pr79154-simd.f90: Likewise.
	* gfortran.dg/gomp/pure-1.f90: New test.
	* gfortran.dg/gomp/pure-2.f90: New test.
	* gfortran.dg/gomp/pure-3.f90: New test.
	* gfortran.dg/gomp/pure-4.f90: New test.