Add speculative clang-interpreter test

Let's see how far this gets on the build servers. The application requires
native JIT and uses the C standard library, but hopefully we can get this
tested on at least some configurations.

Taking a lead from the clang-format tests, we'll just expect the executable to
get picked up from the build output path for now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207950 91177308-0d34-0410-b5e6-96231b3b80d8
Update lit.cfg to support the clang-interpreter test from r207950

Performs behind-the-scenes RUN line substitution similarly to what's done with
clang-check and clang-format to ensure the executable is found.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@207951 91177308-0d34-0410-b5e6-96231b3b80d8