[lldb][NFCI] Change return type of REPL::GetSourceFileBasename

These don't really need to be in the ConstString StringPool. I've
changed the return type to StringRef because on llvm.org and downstream
in the swift fork, this returns a constant value. We could change it to
return a std::string or something else if it needs to be able to change
between calls.

Differential Revision: https://reviews.llvm.org/D151962