[flang] Fix crash from PDT component init in module file

[klausler]

Summary

Semantics now needs to preserve the parse trees from module files,
in case they contain parameterized derived type definitions with
component initializers that may require re-analysis during PDT
instantiation. Save them in the SemanticsContext.

[clementval]

LGTM

flang/include/flang/Semantics/semantics.h

public:
 
   // Locate and process the contents of a built-in module on demand
   Scope *GetBuiltinModule(const char *name);
 
   // Defines builtinsScope_ from the __Fortran_builtins module
   void UseFortranBuiltinsModule();
   const Scope *GetBuiltinsScope() const { return builtinsScope_; }
 
+  // Saves a module file's parse tree so that it remains available
+  // during semantics.
+  parser::Program &SaveParseTree(parser::Program &&);
+
 private:
   void CheckIndexVarRedefine(
       const parser::CharBlock &, const Symbol &, parser::MessageFixedText &&);
   void CheckError(const Symbol &);
 
   const common::IntrinsicTypeDefaultKinds &defaultKinds_;
   const common::LanguageFeatureControl languageFeatures_;
   parser::AllCookedSources &allCookedSources_;
struct IndexVarInfo {
     parser::CharBlock location;
     IndexVarKind kind;
   };
   std::map<SymbolRef, const IndexVarInfo, SymbolAddressCompare>
       activeIndexVars_;
   UnorderedSymbolSet errorSymbols_;
   std::set<std::string> tempNames_;
   const Scope *builtinsScope_{nullptr}; // module __Fortran_builtins
+  std::list<parser::Program> modFileParseTrees_;
 };
 
 class Semantics {
 public:
   explicit Semantics(SemanticsContext &context, parser::Program &program,
       bool debugModuleWriter = false)
       : context_{context}, program_{program} {
     context.set_debugModuleWriter(debugModuleWriter);

flang/lib/Semantics/mod-file.cpp

Scope *ModFileReader::Read(const SourceName &name,
   CHECK(sourceFile);
   if (!VerifyHeader(sourceFile->content())) {
     Say(name, ancestorName, "File has invalid checksum: %s"_warn_en_US,
         sourceFile->path());
     return nullptr;
   }
   llvm::raw_null_ostream NullStream;
   parsing.Parse(NullStream);
-  auto &parseTree{parsing.parseTree()};
+  std::optional<parser::Program> &parsedProgram{parsing.parseTree()};
   if (!parsing.messages().empty() || !parsing.consumedWholeFile() ||
-      !parseTree) {
+      !parsedProgram) {
     Say(name, ancestorName, "Module file is corrupt: %s"_err_en_US,
         sourceFile->path());
     return nullptr;
   }
+  parser::Program &parseTree{context_.SaveParseTree(std::move(*parsedProgram))};
   Scope *parentScope; // the scope this module/submodule goes into
   if (!isIntrinsic.has_value()) {
     for (const auto &dir : context_.intrinsicModuleDirectories()) {
       if (sourceFile->path().size() > dir.size() &&
           sourceFile->path().find(dir) == 0) {
         isIntrinsic = true;
         break;
       }
     }
   }
   Scope &topScope{isIntrinsic.value_or(false) ? context_.intrinsicModulesScope()
                                               : context_.globalScope()};
   if (!ancestor) {
     parentScope = &topScope;
-  } else if (std::optional<SourceName> parent{GetSubmoduleParent(*parseTree)}) {
+  } else if (std::optional<SourceName> parent{GetSubmoduleParent(parseTree)}) {
     parentScope = Read(*parent, false /*not intrinsic*/, ancestor, silent);
   } else {
     parentScope = ancestor;
   }
   auto pair{parentScope->try_emplace(name, UnknownDetails{})};
   if (!pair.second) {
     return nullptr;
   }
   Symbol &modSymbol{*pair.first->second};
   modSymbol.set(Symbol::Flag::ModFile);
-  ResolveNames(context_, *parseTree, topScope);
+  ResolveNames(context_, parseTree, topScope);
   CHECK(modSymbol.has<ModuleDetails>());
   CHECK(modSymbol.test(Symbol::Flag::ModFile));
   if (isIntrinsic.value_or(false)) {
     modSymbol.attrs().set(Attr::INTRINSIC);
   }
   return modSymbol.scope();
 }
 

flang/lib/Semantics/semantics.cpp

void SemanticsContext::UseFortranBuiltinsModule() {
   if (builtinsScope_ == nullptr) {
     builtinsScope_ = GetBuiltinModule("__fortran_builtins");
     if (builtinsScope_) {
       intrinsics_.SupplyBuiltins(*builtinsScope_);
     }
   }
 }
 
+parser::Program &SemanticsContext::SaveParseTree(parser::Program &&tree) {
+  return modFileParseTrees_.emplace_back(std::move(tree));
+}
+
 bool Semantics::Perform() {
   // Implicitly USE the __Fortran_builtins module so that special types
   // (e.g., __builtin_team_type) are available to semantics, esp. for
   // intrinsic checking.
   if (!program_.v.empty()) {
     const auto *frontModule{std::get_if<common::Indirection<parser::Module>>(
         &program_.v.front().u)};
     if (frontModule &&