[flang] Add -f[no-]vectorize flags (#119718)
This patch adds the -fvectorize and -fno-vectorize flags to flang.
Note that this also changes the behaviour of `flang -fc1` to match that
of `clang -cc1`, which is that vectorization is only enabled in the
presence of the `-vectorize-loops` flag.
Additionally, this patch changes the behaviour of the default
optimisation levels to match clang, such that vectorization only happens
at the same levels as it does there.
This patch is in draft while I write an RFC to discuss the above two
changes.
