[LLDB][REPL] Destroy the repl instances correctly

This change ensures that the REPL map inside Target is destroyed correctly when `Target::Destroy()` is invoked. Not doing so results in the REPL not finishing its job when the debugger is initialized in REPL mode reading from a file. In my particular case, my REPL doesn't print the stdout of the last expression in the input file unless I include this change.

Differential Revision: https://reviews.llvm.org/D149180