[lldb/Target] Slide source display for artificial locations at function start

It can happen that a line entry reports that some source code is located
at line 0. In DWARF, line 0 is a special location which indicates that
code has no 1-1 mapping with source.

When stopping in one of those artificial locations, lldb doesn't know which
line to display and shows the beginning of the file instead.

This patch mitigates this behaviour by checking if the current symbol context
of the line entry has a matching function, in which case, it slides the
source listing to the start of that function.

This patch also shows the user a warning explaining why lldb couldn't
show sources at that location.

rdar://83118425

Differential Revision: https://reviews.llvm.org/D115313



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