[flang] Support implicit global external as procedure pointer target

[klausler]

Summary

A name that has been used to reference an undeclared global external
procedure should be accepted as the target of a procedure pointer
assignment statement.

Fixes llvm-test-suite/Fortran/gfortran/regression/proc_ptr_45.f90.

[clementval]

LGTM

flang/lib/Semantics/resolve-names.cpp

           &MakeSymbol(InclusiveScope(), name.source, Attrs{Attr::INTRINSIC});
     } else if (const auto ppcBuiltinScope =
                    currScope().context().GetPPCBuiltinsScope()) {
       // Check if it is a builtin from the predefined module
       symbol = FindSymbol(*ppcBuiltinScope, name);
-      if (!symbol)
+      if (!symbol) {
         symbol = &MakeSymbol(context().globalScope(), name.source, Attrs{});
+      }
     } else {
       symbol = &MakeSymbol(context().globalScope(), name.source, Attrs{});
     }
     Resolve(name, *symbol);
     ConvertToProcEntity(*symbol);

           IsUplevelReference(*symbol)) {
         MakeHostAssocSymbol(*name, *symbol);
       }
       return false;
     }
+    // Can also reference a global external procedure here
+    if (auto it{context().globalScope().find(name->source)};
+        it != context().globalScope().end()) {
+      Symbol &global{*it->second};
+      if (IsProcedure(global)) {
+        Resolve(*name, global);
+        return false;
+      }
+    }
   }
   Walk(expr);
   return false;
 }
 void ResolveNamesVisitor::Post(const parser::Designator &x) {

flang/test/Semantics/assign09.f90

   call sub1(sqrt) ! ok
   call sub2(sqrt) ! ok
   !ERROR: Actual procedure argument has interface incompatible with dummy argument 'p=': incompatible dummy argument #1: incompatible dummy data object types: REAL(4) vs INTEGER(4)
   call sub3(sqrt)
 
+  print *, implicitExtFunc()
+  call implicitExtSubr
+  noInterfaceProcPtr => implicitExtFunc ! ok
+  noInterfaceProcPtr => implicitExtSubr ! ok
   noInterfaceProcPtr => noInterfaceExternal ! ok
   realToRealProcPtr => noInterfaceExternal ! ok
   intToRealProcPtr => noInterfaceExternal !ok
   call sub1(noInterfaceExternal) ! ok
   !WARNING: Actual procedure argument has an implicit interface which is not known to be compatible with dummy argument 'p=' which has an explicit interface