[flang] add nested DEC STRUCTURE in DerivedTypeDetails component names

[jeanPerier]

Summary

Currently, when a (legacy) DEC structure contained other DEC structure
declarations, the related component names were not added to the
containing DerivedTypeDetails component_names. This lead to bugs in
later phase when visiting the components (like in when lowering the
type to FIR/MLIR).

When an EntityDecl is visited and the scope is a DEC structure, add
the entity to the component names of this DEC structure.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Lower/ConvertExprToHLFIR.cpp

assert(
         !componentSym.test(Fortran::semantics::Symbol::Flag::ParentComp) &&
         "parent components are skipped and must not reach visitComponentImpl");
     partInfo.componentName = componentSym.name().ToString();
     auto recordType =
         hlfir::getFortranElementType(baseType).cast<fir::RecordType>();
     if (recordType.isDependentType())
       TODO(getLoc(), "Designate derived type with length parameters in HLFIR");
     mlir::Type fieldType = recordType.getType(partInfo.componentName);
+    assert(fieldType && "component name is not known");
     mlir::Type fieldBaseType =
         hlfir::getFortranElementOrSequenceType(fieldType);
     partInfo.componentShape = genComponentShape(componentSym, fieldBaseType);
 
     mlir::Type fieldEleType = hlfir::getFortranElementType(fieldBaseType);
     if (fir::isRecordWithTypeParameters(fieldEleType))
       TODO(loc,
            "lower a component that is a parameterized derived type to HLFIR");

flang/lib/Semantics/resolve-names.cpp

void DeclarationVisitor::Post(const parser::EntityDecl &x) {
   if (const auto &init{std::get<std::optional<parser::Initialization>>(x.t)}) {
     ConvertToObjectEntity(symbol) || ConvertToProcEntity(symbol);
     symbol.set(
         Symbol::Flag::EntryDummyArgument, false); // forestall excessive errors
     Initialization(name, *init, false);
   } else if (attrs.test(Attr::PARAMETER)) { // C882, C883
     Say(name, "Missing initialization for parameter '%s'"_err_en_US);
   }
+  if (auto *scopeSymbol{currScope().symbol()})
+    if (auto *details{scopeSymbol->detailsIf<DerivedTypeDetails>()})
+      if (details->isDECStructure())
+        details->add_component(symbol);
 }
 
 void DeclarationVisitor::Post(const parser::PointerDecl &x) {
   const auto &name{std::get<parser::Name>(x.t)};
   if (const auto &deferredShapeSpecs{
           std::get<std::optional<parser::DeferredShapeSpecList>>(x.t)}) {
     CHECK(arraySpec().empty());
     BeginArraySpec();

flang/test/Semantics/struct02.f90

+! Test component name resolution with nested legacy DEC structures.
+!RUN: %flang_fc1 -fdebug-dump-symbols %s | FileCheck %s
+
+  structure /a/
+    integer :: a_first_comp
+    structure /b/ b1, b2(100)
+      integer :: i
+    end structure
+    structure /c/ z
+      integer :: i
+      structure /d/ d1, d2(10)
+        real :: x
+      end structure
+    end structure
+    integer :: a_last_comp
+  end structure
+end
+! CHECK:    /a/: DerivedType sequence components: a_first_comp,b1,b2,z,a_last_comp
+! CHECK:    /b/: DerivedType sequence components: i
+! CHECK:    /c/: DerivedType sequence components: i,d1,d2
+! CHECK:    /d/: DerivedType sequence components: x