[flang][NFC] Verify C781 from - Clause 7 constraint checks for f18.

[PeteSteinfeld]

Builds, tests, and looks good.

Thanks for working on this!

flang/lib/Semantics/resolve-names.cpp

bool AttrsVisitor::HaveAttrConflict(Attr attrName, Attr attr1, Attr attr2) {
   }
   return false;
 }
 // C759, C1543
 bool AttrsVisitor::IsConflictingAttr(Attr attrName) {
   return HaveAttrConflict(attrName, Attr::INTENT_IN, Attr::INTENT_INOUT) ||
       HaveAttrConflict(attrName, Attr::INTENT_IN, Attr::INTENT_OUT) ||
       HaveAttrConflict(attrName, Attr::INTENT_INOUT, Attr::INTENT_OUT) ||
-      HaveAttrConflict(attrName, Attr::PASS, Attr::NOPASS) ||
+      HaveAttrConflict(attrName, Attr::PASS, Attr::NOPASS) || // C781
       HaveAttrConflict(attrName, Attr::PURE, Attr::IMPURE) ||
       HaveAttrConflict(attrName, Attr::PUBLIC, Attr::PRIVATE) ||
       HaveAttrConflict(attrName, Attr::RECURSIVE, Attr::NON_RECURSIVE);
 }
 bool AttrsVisitor::CheckAndSet(Attr attrName) {
   CHECK(attrs_);
   if (IsConflictingAttr(attrName) || IsDuplicateAttr(attrName)) {
     return false;

flang/test/Semantics/bindings01.f90

 ! RUN: %S/test_errors.sh %s %t %f18
 ! Confirm enforcement of constraints and restrictions in 7.5.7.3
-! and C733, C734 and C779, C780, C781, C782, C783, C784, and C785.
+! and C733, C734 and C779, C780, C782, C783, C784, and C785.
 
 module m
   !ERROR: An ABSTRACT derived type must be extensible
   type, abstract, bind(c) :: badAbstract1
   end type
   !ERROR: An ABSTRACT derived type must be extensible
   type, abstract :: badAbstract2
     sequence

flang/test/Semantics/resolve80.f90

contains
     procedure(subDeferred), deferred, public, deferred :: deferredBinding
     !WARNING: Attribute 'NON_OVERRIDABLE' cannot be used more than once
     procedure, non_overridable, public, non_overridable :: subNon_overridable;
     !WARNING: Attribute 'NOPASS' cannot be used more than once
     procedure(subNopass), nopass, deferred, nopass :: nopassBinding
     !WARNING: Attribute 'PASS' cannot be used more than once
     procedure(subPass), pass, deferred, pass :: passBinding
     !ERROR: Attributes 'PASS' and 'NOPASS' conflict with each other
-    procedure(subPassNopass), pass, deferred, nopass :: passNopassBinding
+    procedure(subPassNopass), pass, deferred, nopass :: passNopassBinding  ! C781
   end type boundProcType
 
 contains
     subroutine subPublic(x)
       class(boundProcType), intent(in) :: x
     end subroutine subPublic
 
     subroutine subPrivate(x)