[flang] Lower 2-argument F2008 atan intrinsic
Implement lowering and testing of the atan(y,x) form of the atan
intrinsic. Since the standard specifies that the behavior of this form
is identical to the atan2 intrinsic, everything is lowered to atan2.
In addition to tests for the atan(y,x) intrinsic, tests for the atan(x)
intrinsic were also added/updated.
