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