[lldb] Improve corefile saving ergonomics

This patch improves the way the user can save the process state into a
corefile by adding completion handler that would provide tab completion
for the corefile path and also resolves the corefile path to expand
relative path.

Differential Revision: https://reviews.llvm.org/D152842



Signed-off-by: Med Ismail Bennani <ismail@bennani.ma>