[clang-repl] Enable debugging of JIT-ed code.

This change follows llvm/llvm-project@21b5ebd and makes use of the jitlink
infrastructure. In order to use this feature inside lldb one needs to run the
lldb command: settings set plugin.jit-loader.gdb.enable on

This works currently only on Darwin since jitlink is not a default ELF/x86-64
backend yet.

Differential revision: https://reviews.llvm.org/D148481