[flang] _FortranAExit does not have [[noreturn]] semantic

[foxtran]

For the following code:

integer function KOHb_exit(status)
  integer, intent(in) :: status
  if (status > 0) call exit(status) ! actually, _FortranAExit
  KOHb_exit = 0
  do i = 1, status
    print '(A,I0)', "KOHb #", i
  end do
end function KOHb_exit

one would expect that this code will never have print statement in ASM at high level of optimizations, since execution after exit is not possible. However, current LLVM flang generates print statements at -O3:
https://godbolt.org/z/EzK3nWfrq

For error stop (as well as stop), the code looks cool:
https://godbolt.org/z/7bvhf4Ef3

[eugeneepshteyn]

Right, need to emit fir.unreachable when emitting exit.

[eugeneepshteyn]

Also, need to document EXIT(), since it's GNU extension: https://gcc.gnu.org/onlinedocs/gfortran/EXIT.html