This patch removes f18, a.k.a. the old driver. It is being replaced
with the new driver, flang-new, which has reached feature parity with
f18 a while ago. This was discussed in  and also in .
With this change, FLANG_BUILD_NEW_DRIVER is no longer needed is deleted
as well. This means that we are making the dependency on Clang permanent
(i.e. it cannot be disabled with a CMake flag).
LIT set-up is updated accordingly. All references to f18 or f18.cpp
are either updated or removed.