[lldb] Change return type of UnixSignals::GetShortName

The short names of each signal name and alias only exist as ConstStrings
in this one scenario. For example, GetShortName("SIGHUP") will just give
you "HUP". There's not a good reason the string "HUP" needs to be in the
ConstString StringPool, and that's true for just about every signal
name.

Differential Revision: https://reviews.llvm.org/D152582