Forbid driver use in Sema tests

This ports the last Sema tests over to use the frontend directly, and adds a
local lit substitution to disable inappropriate %clang usage under this
directory.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@199348 91177308-0d34-0410-b5e6-96231b3b80d8