[lldb][NFCI] Remove use of ConstString from UnixSignals

The majority of UnixSignals strings are static in the sense that they do
not change. The overwhelming majority of these strings are string
literals. Using ConstString to manage their lifetime does not make
sense. The only exception to this is one of the subclasses of
UnixSignals, for which I have created a StringSet local to that file
which will guarantee the lifetimes of these StringRefs.

As for the other benefits of ConstString, string uniqueness is not a
concern (as many of them are already string literals) and comparing
signal names and aliases should not be a hot path.

Differential Revision: https://reviews.llvm.org/D159011