Use clause locations in OpenACC nested reduction warnings

Since the Fortran front-end now sets the clause locations correctly, we can
emit warnings with more precise locations if we encounter conflicting
operations for a variable in reduction clauses.

2019-12-10  Frederik Harwath  <frederik@codesourcery.com>

gcc/
	* omp-low.c (scan_omp_for): Use clause location in warning.

From-SVN: r279168