[lldb/Interpreter] Fix language detection for the REPL InitFile

[lldb/Interpreter] Fix language detection for the REPL InitFile

Previously, before loading the REPL language-specific init file, lldb
checked the selected target language in which case it returned an unknown
language type with the REPL target.

Instead, the patch calls `Language::GetLanguagesSupportingREPLs` and
look for the first element of that set. In case lldb was not configured
with a REPL language, then, it will just stop sourcing the REPL init
file and fallback to the original logic (continuing with the default
init file).

rdar://65836048

Differential Revision: https://reviews.llvm.org/D87076

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