[flang] Add options to control IMPLICIT NONE

Add `-fimplicit-none-type-always` to treat each specification-part
like it has `IMPLICIT NONE`. This is helpful for enforcing good Fortran
programming practices. We might consider something similar for
`IMPLICIT NONE(EXTERNAL)` as well.

Add `-fimplicit-none-type-never` to ignore occurrences of `IMPLICIT NONE`
and `IMPLICIT NONE(TYPE)`. This is to handle cases like the one below,
which violates the standard but it accepted by some compilers:
```
subroutine s(a, n)
  implicit none
  real :: a(n)
  integer :: n
end
```

Differential Revision: https://reviews.llvm.org/D85363