[flang] Silence inappropriate error message

A recent patch added better compatibility checking for actual
procedure arguments, but it has led to a few failures in the Fujitsu
Fortran test suite in cases of NULL() actual arguments being
associated with dummy procedure pointers.  As is the case with
dummy data pointers, these must always be accepted.

Fixes Fujitsu Fortran test cases 0249_0023 through 0028 and
0387_0047.

Author: klausler

flang/lib/Semantics/expression.cpp

@@ -2536,6 +2536,15 @@ static bool CheckCompatibleArgument(bool isElemental,
             return false;
           },
           [&](const characteristics::DummyProcedure &dummy) {
+            if ((dummy.attrs.test(
+                     characteristics::DummyProcedure::Attr::Optional) ||
+                    dummy.attrs.test(
+                        characteristics::DummyProcedure::Attr::Pointer)) &&
+                IsBareNullPointer(expr)) {
+              // NULL() is compatible with any dummy pointer
+              // or optional dummy procedure.
+              return true;
+            }
             if (!expr || !IsProcedurePointerTarget(*expr)) {
               return false;
             }