clang-interpreter: use LLVM interpreter if JIT is unavailable

Update the strategy in r212083 to try JIT first and otherwise fall back to the
interpreter. This gives the best of both worlds and still builds fine with no
targets enabled.

Requires supporting changes from LLVM r212086.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@212087 91177308-0d34-0410-b5e6-96231b3b80d8