This patch adds Fortran support for OMP 5.1 "canonical loop nest form" 
and non-rectangular loops.  The C/C++ and middle-end support is already 
present except for some missing constraint checks in the gimplifier, 
which I've added here.  There's still a TODO with respect to the 
not-yet-implemented TILE construct.

Is this OK for stage 1 when the time comes?

-Sandra