[flang] Expunge bogus semantic check for ELEMENTAL without dummies

[klausler]

Summary

Semantics refuses valid ELEMENTAL subprograms without dummy arguments,
but there's no such constraint in the standard; indeed, subclause
15.8.2 discusses the meaning of calls to ELEMENTAL functions with
arguments. Remove the check and its test.

[PeteSteinfeld]

All builds, tests, and looks good.

flang/lib/Semantics/check-declarations.cpp

if (error) {
         }
       }
     }
   }
   if (symbol.attrs().test(Attr::ELEMENTAL)) {
     // See comment on the similar check in CheckProcEntity()
     if (details.isDummy()) {
       messages_.Say("A dummy procedure may not be ELEMENTAL"_err_en_US);
-    } else if (details.dummyArgs().empty()) {
-      messages_.Say(
-          "An ELEMENTAL subprogram must have at least one dummy argument"_err_en_US);
     } else {
       for (const Symbol *dummy : details.dummyArgs()) {
         if (!dummy) { // C15100
           messages_.Say(
               "An ELEMENTAL subroutine may not have an alternate return dummy argument"_err_en_US);
         }
       }
     }

flang/test/Semantics/elemental01.f90

 ! RUN: %python %S/test_errors.py %s %flang_fc1
 ! Tests ELEMENTAL subprogram constraints C15100-15102
 
-!ERROR: An ELEMENTAL subprogram must have at least one dummy argument
-elemental integer function noargs
-  noargs = 1
-end function
-
 !ERROR: An ELEMENTAL subroutine may not have an alternate return dummy argument
 elemental subroutine altret(*)
 end subroutine
 
 elemental subroutine arrarg(a)
   !ERROR: A dummy argument of an ELEMENTAL procedure must be scalar
   real, intent(in) :: a(1)
 end subroutine