ExecutionEngine::create(): fix interpreter fallback when JIT is unavailable

ForceInterpreter=false shouldn't disable the interpreter completely because it
can still be necessary to interpret if the target doesn't support JIT.

No obvious way to test this in LLVM, but this matches what
LLVMCreateExecutionEngineForModule() does and fixes the clang-interpreter
example in the clang source tree which uses the ExecutionEngine.

llvm-svn: 212086