[flang] fulfill -Msave/-fno-automatic in main programs too

`semantics::IsSaved()` was not applying -Msave/-fno-automatic for main programs.
This caused issues since lowering relies on it to allocate static
variables. This did not match nvfortran/gfortran behaviors where
-fno-automatic/-Msave control the static allocation of scalars in
main programs.

Some program may rely on main program scalars to be statically allocated in
bss (and therefore initialized to zero) with -Msave/-fno-automatic
flags.

Differential Revision: https://reviews.llvm.org/D121603