[lldb] Provide a getter for m_materializer_up in LLVMUserExpression instead of relying on it being accessible.

[teemperor]

Summary

Motivated by Swift using the materializer in a few places which requires us to add this getter ourselves.
We also need a setter, but let's keep this minimal to unblock the downstream reverts in Swift.

[davide]

LGTM.

lldb/include/lldb/Expression/Expression.h

public:
   /// Return the function name that should be used for executing the
   /// expression.  Text() should contain the definition of this function.
   virtual const char *FunctionName() = 0;
 
   /// Return the language that should be used when parsing.  To use the
   /// default, return eLanguageTypeUnknown.
   virtual lldb::LanguageType Language() { return lldb::eLanguageTypeUnknown; }
 
+  /// Return the Materializer that the parser should use when registering
+  /// external values.
+  virtual Materializer *GetMaterializer() { return nullptr; }
+
   /// Return the desired result type of the function, or eResultTypeAny if
   /// indifferent.
   virtual ResultType DesiredResultType() { return eResultTypeAny; }
 
   /// Flags
 
   /// Return true if validation code should be inserted into the expression.
   virtual bool NeedsValidation() = 0;

lldb/include/lldb/Expression/LLVMUserExpression.h

public:
   bool FinalizeJITExecution(
       DiagnosticManager &diagnostic_manager, ExecutionContext &exe_ctx,
       lldb::ExpressionVariableSP &result,
       lldb::addr_t function_stack_bottom = LLDB_INVALID_ADDRESS,
       lldb::addr_t function_stack_top = LLDB_INVALID_ADDRESS) override;
 
   bool CanInterpret() override { return m_can_interpret; }
 
+  Materializer *GetMaterializer() override { return m_materializer_up.get(); }
+
   /// Return the string that the parser should parse.  Must be a full
   /// translation unit.
   const char *Text() override { return m_transformed_text.c_str(); }
 
   lldb::ModuleSP GetJITModule() override;
 
 protected:
   lldb::ExpressionResults

lldb/source/Plugins/ExpressionParser/Clang/ClangUserExpression.cpp

bool ClangUserExpression::Parse(DiagnosticManager &diagnostic_manager,
   //
 
   m_materializer_up.reset(new Materializer());
 
   ResetDeclMap(exe_ctx, m_result_delegate, keep_result_in_memory);
 
   auto on_exit = llvm::make_scope_exit([this]() { ResetDeclMap(); });
 
-  if (!DeclMap()->WillParse(exe_ctx, m_materializer_up.get())) {
+  if (!DeclMap()->WillParse(exe_ctx, GetMaterializer())) {
     diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "current process state is unsuitable for expression parsing");
     return false;
   }
 
   if (m_options.GetExecutionPolicy() == eExecutionPolicyTopLevel) {
     DeclMap()->SetLookupsEnabled(true);
bool ClangUserExpression::Complete(ExecutionContext &exe_ctx,
   //
 
   m_materializer_up.reset(new Materializer());
 
   ResetDeclMap(exe_ctx, m_result_delegate, /*keep result in memory*/ true);
 
   auto on_exit = llvm::make_scope_exit([this]() { ResetDeclMap(); });
 
-  if (!DeclMap()->WillParse(exe_ctx, m_materializer_up.get())) {
+  if (!DeclMap()->WillParse(exe_ctx, GetMaterializer())) {
     diagnostic_manager.PutString(
         eDiagnosticSeverityError,
         "current process state is unsuitable for expression parsing");
 
     return false;
   }
 
   if (m_options.GetExecutionPolicy() == eExecutionPolicyTopLevel) {