Previously this took a reference to a map and returned a bool to say
whether it succeeded. We can return a StringMap instead, as all callers
but 1 simply iterated the map if the bool was true, and passed in empty
maps as the starting point.
lldb's lit-cpuid did specifically check whether the call failed, but due
to the way the x86 routines work this works out the same as checking if
the returned map is empty.
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