[flang] Allow pointers to non-sequence types in sequence types

[klausler]

Summary

Derived types with SEQUENCE must have data components of sequence
types; but this rule is relaxed as common an extension in the case of
pointer components, whose targets' types are not really relevant
to the implementation requirements of sequence types.

[klausler]

Rebase (fixing build issue).

[PeteSteinfeld]

All builds, tests, and looks good.

flang/docs/Extensions.md

 * Objects in blank COMMON may be initialized.
 * Multiple specifications of the SAVE attribute on the same object
   are allowed, with a warning.
+* A `POINTER` component's type need not be a sequence type when
+  the component appears in a derived type with `SEQUENCE`.
+  (This case should probably be an exception to constraint C740 in
+  the standard.)
 
 ### Extensions supported when enabled by options
 

flang/include/flang/Common/Fortran-features.h

     OldLabelDoEndStatements, LogicalIntegerAssignment, EmptySourceFile,
     ProgramReturn, ImplicitNoneTypeNever, ImplicitNoneTypeAlways,
     ForwardRefDummyImplicitNone, OpenAccessAppend, BOZAsDefaultInteger,
-    DistinguishableSpecifics, DefaultSave)
+    DistinguishableSpecifics, DefaultSave, PointerInSeqType)
 
 using LanguageFeatures = EnumSet<LanguageFeature, LanguageFeature_enumSize>;
 

flang/lib/Semantics/resolve-names.cpp

   if (derivedTypeInfo_.sequence) { // C740
     if (const auto *declType{GetDeclTypeSpec()}) {
       if (!declType->AsIntrinsic() && !declType->IsSequenceType()) {
-        Say("A sequence type data component must either be of an"
-            " intrinsic type or a derived sequence type"_err_en_US);
+        if (GetAttrs().test(Attr::POINTER) &&
+            context().IsEnabled(common::LanguageFeature::PointerInSeqType)) {
+          if (context().ShouldWarn(common::LanguageFeature::PointerInSeqType)) {
+            Say("A sequence type data component that is a pointer to a non-sequence type is not standard"_en_US);
+          }
+        } else {
+          Say("A sequence type data component must either be of an intrinsic type or a derived sequence type"_err_en_US);
+        }
       }
     }
   }

flang/test/Semantics/resolve31.f90

     class(*), allocatable :: typeStarField
     !ERROR: A sequence type data component must either be of an intrinsic type or a derived sequence type
     type(plainType) :: testField1
+    !Pointers are ok as an extension
+    type(plainType), pointer :: testField1p
     type(sequenceType) :: testField2
     procedure(real), pointer, nopass :: procField
   end type testType