[lldb/Target] Refine source display warning for artificial locations (NFC)

This is a post-review update for D115313, to rephrase source display
warning messages for artificial locations, making them more
understandable for the end-user.

Differential Revision: https://reviews.llvm.org/D115461



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>