clang-interpreter: interpret instead of JITing

Fixes the build when no targets are selected, or no native target is built.

This also better matches up with the description/title of the example and
demonstrates how clang can be used to run C++ on constrained environments
without file IO or executable memory permissions (e.g. iOS apps).

A comment is added explaining how to extend the demo with JIT support as
needed.
driver: add link dependency on CodeGen

Fix the build when no targets are enabled. This dependency is incurred by two
unfortunate entries in LinkAllPasses.h included from cc1_main.cpp:

  llvm::createJumpInstrTablesPass();
  llvm::createCodeGenPreparePass();