openmp: Improve Fortran Diagnostics for Linear Clause

openmp: Improve Fortran Diagnostics for Linear Clause

This patch improves diagnostics for the linear clause,
providing a more accurate and intuitive recommendation
for remediation if the deprecated syntax is used.
Additionally updates the relevant test to reflect the
changed verbiage of the warning.

gcc/fortran/ChangeLog:

	* openmp.cc (gfc_match_omp_clauses): New diagnostic logic.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/pr84418-1.f90: Fix verbiage of
	dg-warning to reflect updated warning.