[flang] Detect call to abstract interface

[tskeith]

Summary

A subroutine call or function reference to an abstract interface is
not legal.

flang/lib/Semantics/resolve-names.cpp

if (convertedToProcEntity && !symbol->attrs().test(Attr::EXTERNAL) &&
       symbol->attrs().set(Attr::INTRINSIC);
       // 8.2(3): ignore type from intrinsic in type-declaration-stmt
       symbol->get<ProcEntityDetails>().set_interface(ProcInterface{});
     }
     if (!SetProcFlag(name, *symbol, flag)) {
       return; // reported error
     }
     CheckImplicitNoneExternal(name.source, *symbol);
-    if (IsProcedure(*symbol) || symbol->has<DerivedTypeDetails>() ||
+    if (symbol->has<SubprogramDetails>() &&
+        symbol->attrs().test(Attr::ABSTRACT)) {
+      Say(name, "Abstract interface '%s' may not be called"_err_en_US);
+    } else if (IsProcedure(*symbol) || symbol->has<DerivedTypeDetails>() ||
         symbol->has<ObjectEntityDetails>() ||
         symbol->has<AssocEntityDetails>()) {
       // Symbols with DerivedTypeDetails, ObjectEntityDetails and
       // AssocEntityDetails are accepted here as procedure-designators because
       // this means the related FunctionReference are mis-parsed structure
       // constructors or array references that will be fixed later when
       // analyzing expressions.
     } else if (symbol->test(Symbol::Flag::Implicit)) {

flang/test/Semantics/resolve20.f90

module m
   type, abstract :: t1
     integer :: i
   contains
     !ERROR: 'proc' must be an abstract interface or a procedure with an explicit interface
     !ERROR: Procedure component 'p1' has invalid interface 'proc'
     procedure(proc), deferred :: p1
   end type t1
 
+  abstract interface
+    function f()
+    end function
+  end interface
+
 contains
   subroutine bar
   end subroutine
+  subroutine test
+    !ERROR: Abstract interface 'foo' may not be called
+    call foo()
+    !ERROR: Abstract interface 'f' may not be called
+    x = f()
+  end subroutine
 end module