Option for rain-to-snow to immediately run off in some regions

Up until now: When repartition_rain_snow is .true. (which is the default
for CLM5), rain that falls when the near-surface temperature is cold is
converted to snow. This repartitioning was put in place for two
reasons: (1) Downscaling to elevation classes: changing the balance
between rain and snow for different elevation classes; (2) Correcting
problems in CAM. However, members of the Land Ice Working Group would
like to change this behavior so that, when CAM produces cold-temperature
rain, this rain immediately runs off rather than being converted to
snow. The purpose of this is to reduce the too-high SMB over portions of
Greenland in CESM2 coupled runs (which results in part from CAM's
generation of liquid precipitation despite very cold temperatures).

This new behavior is implemented in a glacier region-specific manner,
based on a new namelist flag, glacier_region_rain_to_snow_behavior. It
is not at all ideal to make this aspect of the physics differ by region,
but this has been requested by members of the Land Ice Working Group in
order to address biases over Greenland while having minimal impact on
the climate (so that the climate can stay very similar to that of the
official CMIP6 runs). Note that, unlike other glacier region-specific
behaviors, this one applies to all landunits, not just glaciers. This
also seems a bit non-ideal, but we want the physics to be the same for
all landunit types in a given region, and we also want this behavior to
apply to vegetated columns because they are used for glacial
inception (and we want this alternate behavior to apply to glacial
inception, too, in order to decrease some instances of inception).

The justification for this new physics is: In the case of (1) above: If
CAM is generating rain at a given elevation / temperature, that doesn't
necessarily imply that an equal water equivalent of snow would be
generated at a higher elevation / lower temperature: indeed, in reality,
there might not be any precipitation falling at that higher elevation /
lower temperature. In the case of (2) above: There seem to be problems
with CAM's microphysics that cause it to produce too much rain when
temperatures are very cold; it seems (at least to some people) equally
justifiable to throw this cold rain away (by sending it to the ocean as
runoff) as it is to convert this cold rain to snow.