[flang] Keep PURE in IEEE functions

[clementval]

Summary

PURE keyword should be kept in __fortran_ieee_exceptions.f90
and ieee_arithmetic.f90 and not removed as done in
https://reviews.llvm.org/D128431

[vdonaldson]

Looks good, thanks!

flang/module/__fortran_ieee_exceptions.f90

#undef IEEE_SET_HALTING_MODE_L
   interface
     subroutine ieee_set_status(status)
       import ieee_status_type
       type(ieee_status_type), intent(in) :: status
     end subroutine ieee_set_status
   end interface
 
 #define IEEE_SUPPORT_FLAG_R(XKIND) \
-  logical function ieee_support_flag_a##XKIND(flag, x); \
+  pure logical function ieee_support_flag_a##XKIND(flag, x); \
     import ieee_flag_type; \
     type(ieee_flag_type), intent(in) :: flag; \
     real(XKIND), intent(in) :: x(..); \
   end function ieee_support_flag_a##XKIND;
   interface ieee_support_flag
-    logical function ieee_support_flag(flag)
+    pure logical function ieee_support_flag(flag)
       import ieee_flag_type
       type(ieee_flag_type), intent(in) :: flag
     end function ieee_support_flag
     SPECIFICS_R(IEEE_SUPPORT_FLAG_R)
   end interface ieee_support_flag
   PRIVATE_R(IEEE_SUPPORT_FLAG)
 #undef IEEE_SUPPORT_FLAG_R
 
   interface
     pure logical function ieee_support_halting(flag)
       import ieee_flag_type
       type(ieee_flag_type), intent(in) :: flag
     end function ieee_support_halting
   end interface
 
 end module __Fortran_ieee_exceptions

flang/module/ieee_arithmetic.f90

 
 #define IEEE_SUPPORT_ROUNDING_R(XKIND) \
   pure logical function ieee_support_rounding_a##XKIND(round_value, x); \
     import ieee_round_type; \
     type(ieee_round_type), intent(in) :: round_value; \
     real(XKIND), intent(in) :: x(..); \
   end function ieee_support_rounding_a##XKIND;
   interface ieee_support_rounding
-    logical function ieee_support_rounding(round_value)
+    pure logical function ieee_support_rounding(round_value)
       import ieee_round_type
       type(ieee_round_type), intent(in) :: round_value
     end function ieee_support_rounding
     SPECIFICS_R(IEEE_SUPPORT_ROUNDING_R)
   end interface ieee_support_rounding
   PRIVATE_R(IEEE_SUPPORT_ROUNDING)
 #undef IEEE_SUPPORT_ROUNDING_R