Let clang-repl link privately against Clang components

First of all, this is the convention: all other tools have their
dependencies private. While it does not have an effect on linking
(there is no linking against executables), it does have an effect
on exporting: having the targets private allows installing the tools
without the libraries in a statically linked build, or a build against
libclang-cpp.so.

Reviewed By: v.g.vassilev

Differential Revision: https://reviews.llvm.org/D122546