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

On llvm.org and all downstream forks that I'm aware of, SignalCodes are
always created from C string literals. They are never compared to
anything so they take up space in the ConstString StringPool for no
tangible benefit.

I've changed the type here to `const llvm::StringLiteral` instead of
using a `StringRef` or a `const char *` to express intent -- These
strings come from constant data whose lifetime is directly tied to that
of the running process (and are thus safe to store).

Differential Revision: https://reviews.llvm.org/D151516