[flang] Switch lowering to use the HLFIR step by default (#72090)

Patch 3/3 of the transition step 1 described in

https://discourse.llvm.org/t/rfc-enabling-the-hlfir-lowering-by-default/72778/7

This patch changes bbc and flang-new driver to use HLFIR lowering by
default.

`-hlfir=false` can be used with bbc and `-flang-deprecated-no-hlfir`
with flang-new to get the previous default lowering behavior, but these
options will only be available for a limited period of time.

If any user needs these options to workaround bugs, they should open an
issue against flang in llvm github repo so that the regression can be
fixed in HLFIR.