[flang][cmake] Enable the new driver by default

With this patch, `FLANG_BUILD_NEW_DRIVER` is set to `On` by default
(i.e. the new driver is enabled). Note that the new driver depends on
Clang and hence with this change you will need to add `clang` to
`LLVM_ENABLE_PROJECTS`.

If you don't want to build the new driver, set `FLANG_BUILD_NEW_DRIVER`
to `Off`. This way you won't be required to include `clang` in
`LLVM_ENABLE_PROJECTS`.

Differential Revision: https://reviews.llvm.org/D101842