This provides a simplified version of the SetReqLen macro when using clang for
static analysis.  Prior to this change, we would see many Idempotent operation
warnings inside this macro due to the common case of calling with arg2 and
arg3 being the same variable.  This has no effect on code produced during
compilation, but it silences a number of false positives in static analysis.

XIPassiveGrab.c:170:5: warning: Assigned value is always the same as the existing value
    SetReqLen(req, num_modifiers, num_modifiers);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from XIPassiveGrab.c:26:
.../nx-X11/lib/X11/Xlibint.h:580:8: note: instantiated from:
            n = badlen; \
              ^
Signed-off-by: Jeremy Huddleston <jeremyhu@apple.com>
Backported-to-NX-by: Ulrich Sibiller <uli42@gmx.de>