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

I'm not convinced that it makes sense for the paths to be ConstStrings. We're
going to be putting them into FileSpecs (which are backed by
ConstStrings, for now) but otherwise there's no need to store them as
ConstStrings upfront.

Differential Revision: https://reviews.llvm.org/D153827