[lldb/interpreter] Add REPL-specific init file

This patch adds the infrastructure to have language specific REPL init
files. It's the foundation work to a following patch that will introduce
Swift REPL init file.

When lldb is launched with the `--repl` option, it will look for a REPL
init file in the home directory and source it. This overrides the
default `~/.lldbinit`, which content might make the REPL behave
unexpectedly. If the REPL init file doesn't exists, lldb will fall back
to the default init file.

rdar://65836048

Differential Revision: https://reviews.llvm.org/D86242



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