[lldb/interpreter] Improve REPL init file compatibility

This patch changes the command interpreter sourcing logic for the REPL
init file. Instead of looking for a arbitrary file name, it standardizes
the REPL init file name to match to following scheme:

                          `.lldbinit-<language>-repl`

This will make the naming more homogenous and the sourcing logic future-proof.

rdar://65836048

Differential Revision: https://reviews.llvm.org/D86987



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