[flang] Catch attempt to misuse an abstract procedure in a generic interface

[klausler]

Summary

A procedure defined in an ABSTRACT INTERFACE may not appear as
a specific procedure in a generic interface.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Semantics/check-declarations.cpp

   GenericKind kind{details.kind()};
   DistinguishabilityHelper helper{context_};
   for (const Symbol &specific : details.specificProcs()) {
+    if (specific.attrs().test(Attr::ABSTRACT)) {
+      if (auto *msg{messages_.Say(generic.name(),
+              "Generic interface '%s' must not use abstract interface '%s' as a specific procedure"_err_en_US,
+              generic.name(), specific.name())}) {
+        msg->Attach(
+            specific.name(), "Definition of '%s'"_en_US, specific.name());
+      }
+    }
     if (const Procedure *procedure{Characterize(specific)}) {
       if (procedure->HasExplicitInterface()) {
         helper.Add(generic, kind, specific, *procedure);

flang/test/Semantics/generic02.f90

 ! RUN: %python %S/test_errors.py %s %flang_fc1
 program test
+  !ERROR: Generic interface 'generic' must not use abstract interface 'abstract' as a specific procedure
   interface generic
     subroutine explicit(n)
       integer, intent(in) :: n
     end subroutine
     procedure implicit
+    procedure abstract
+  end interface
+  abstract interface
+    subroutine abstract
+    end subroutine
   end interface
 !ERROR: Specific procedure 'implicit' of generic interface 'generic' must have an explicit interface
   external implicit