r250293 - Bring back r250262: PS4 toolchain

[Robinson, Paul via cfe-commits]

Nico asks:

+def warn_drv_ps4_force_pic : Warning<
+  "option '%0' was ignored by the PS4 toolchain, using '-fPIC'">,
+  InGroup<OptionIgnored>;

Should this use the existing UnusedCommandLineArgument instead of introducing a new group?

Depends on what kinds of distinctions you want to make, when it comes to how-to-turn-off-warnings.  OptionIgnored, IgnoredOptimizationArgument, InvalidCommandLineArgument, and UnusedCommandLineArgument all seem to be somewhat related; each of the first three has only one warning in the group, maybe they could all be folded into the last one.
Looks like there are other similar warnings that currently have no group but probably could be folded into the group, e.g. warn_c_kext.

Is there guidance/documentation about how groups ought to be used and when it's appropriate to define a new one?  Or is it more like "check around for an existing group that seems to fit"?
Thanks,
--paulr

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