[flang] Allow POINTER attribute statement on procedure interfaces

[klausler]

Summary

A POINTER attribute statement is allowed to add the POINTER attribute
to a procedure entity that has already been declared, e.g. with an
INTERFACE block.

[clementval]

LGTM

flang/lib/Semantics/resolve-names.cpp

     symbol.ReplaceName(name.source);
     EndArraySpec();
   } else {
-    Symbol &symbol{DeclareUnknownEntity(name, Attrs{Attr::POINTER})};
-    symbol.ReplaceName(name.source);
+    HandleAttributeStmt(Attr::POINTER, std::get<parser::Name>(x.t));
   }
 }
 

flang/test/Semantics/assign02.f90

   subroutine s0
     !ERROR: 'p1' may not have both the POINTER and TARGET attributes
     real, pointer :: p1, p3
+    !ERROR: 'p2' may not have both the POINTER and ALLOCATABLE attributes
     allocatable :: p2
     !ERROR: 'sin' may not have both the POINTER and INTRINSIC attributes
     real, intrinsic, pointer :: sin
     target :: p1
-    !ERROR: 'p2' may not have both the POINTER and ALLOCATABLE attributes
     pointer :: p2
     !ERROR: 'a' may not have the POINTER attribute because it is a coarray
     real, pointer :: a(:)[*]