[flang] Allow IMPLICIT NONE(EXTERNAL) with GenericDetails

[FruitClover]

Summary

Restrictions of IMPLICIT NONE(EXTERNAL) prohibits usage of c_associated from
iso_c_binding (with explicit interface) without external definiton - relax
associated check.

[FruitClover]

@clementval thanks for adding reviewers!

[klausler]

Looks okay to me.

[FruitClover]

@klausler Do I need to manually re-trigger CI here? (I see Building status https://reviews.llvm.org/harbormaster/build/226249/ for a long time)

[FruitClover]

Build passed

flang/lib/Semantics/resolve-names.cpp

if (!symbol) {
     bool convertedToProcEntity{ConvertToProcEntity(*symbol)};
     if (convertedToProcEntity && !symbol->attrs().test(Attr::EXTERNAL) &&
         IsIntrinsic(symbol->name(), flag) && !IsDummy(*symbol)) {
       AcquireIntrinsicProcedureFlags(*symbol);
     }
     if (!SetProcFlag(name, *symbol, flag)) {
       return; // reported error
     }
+    if (!symbol->has<GenericDetails>()) {
       CheckImplicitNoneExternal(name.source, *symbol);
+    }
     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<AssocEntityDetails>()) {
       // Symbols with DerivedTypeDetails and AssocEntityDetails are accepted
       // here as procedure-designators because this means the related
       // FunctionReference are mis-parsed structure constructors or array

flang/test/Semantics/implicit12.f90

+! RUN: %python %S/test_errors.py %s %flang_fc1
+use iso_c_binding, only: c_ptr, c_associated
+implicit none(external)
+type (c_ptr) :: cptr
+if (.not. c_associated (cptr)) then
+   return
+end if
+end