clang-interpreter: interpret instead of JITing

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212083 91177308-0d34-0410-b5e6-96231b3b80d8