[clang-repl] Re-implement clang-interpreter as a test case.

The current infrastructure in lib/Interpreter has a tool, clang-repl, very
similar to clang-interpreter which also allows incremental compilation.

This patch moves clang-interpreter as a test case and drops it as conditionally
built example as we already have clang-repl in place.

Differential revision: https://reviews.llvm.org/D107049