[flang] Take character function length into account when testing compatibility

[klausler]

Summary

When a character-valued function is passed as an actual argument, and both
the actual function and the dummy argument have explicit result lengths, take them
into account when testing for compatibility.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Evaluate/characteristics.cpp

         if (whyNot) {
           *whyNot = "function results have distinct constant extents";
         }
-      } else if (!ifaceTypeShape->type().IsTkCompatibleWith(
+      } else if (!ifaceTypeShape->type().IsTkLenCompatibleWith(
                      actualTypeShape->type())) {
         if (whyNot) {
           *whyNot = "function results have incompatible types: "s +

flang/test/Semantics/call25.f90

     character(5), intent(in) :: x
     explicitLength = x
   end function
+  character(6) function badExplicitLength(x)
+    character(5), intent(in) :: x
+    badExplicitLength = x
+  end function
   real function notChar(x)
     character(*), intent(in) :: x
     notChar = 0

   external assumedlength
   character(5) :: assumedlength
   call subr1(explicitLength)
+  !CHECK: error: Actual argument function associated with procedure dummy argument 'f=' has incompatible result type
+  call subr1(badExplicitLength)
   call subr1(assumedLength)
   !CHECK: error: Actual argument function associated with procedure dummy argument 'f=' has incompatible result type
   call subr1(notChar)

   !CHECK: error: Actual argument function associated with procedure dummy argument 'f=' has incompatible result type
   call subr2(notChar)
   call subr3(explicitLength)
+  !CHECK: warning: If the procedure's interface were explicit, this reference would be in error
+  !CHECK: because: Actual argument function associated with procedure dummy argument 'f=' has incompatible result type
+  call subr3(badExplicitLength)
   call subr3(assumedLength)
   !CHECK: warning: If the procedure's interface were explicit, this reference would be in error
   !CHECK: because: Actual argument function associated with procedure dummy argument 'f=' has incompatible result type