[lLDB] Fix crash in TypeSystemClang::GetIndexofChildMemberWithName.

LLDB can crash in TypeSystemClang::GetIndexOfChildMemberWithName, at a
point where it pushes an index onto the child_indexes vector, tries to call
itself recursively, then tries to pop the entry from child_indexes.
The problem is that the recursive call can clear child_indexes, so that
this code ends up trying to pop an already empty vector.  This change saves
the old vector before the push, then restores the saved vector rather than
trying to pop.

Author: cmtice

lldb/source/Plugins/TypeSystem/Clang/TypeSystemClang.cpp

@@ -6754,12 +6754,12 @@ size_t TypeSystemClang::GetIndexOfChildMemberWithName(
           llvm::StringRef field_name = field->getName();
           if (field_name.empty()) {
             CompilerType field_type = GetType(field->getType());
+            std::vector<uint32_t> save_indices = child_indexes;
             child_indexes.push_back(child_idx);
             if (field_type.GetIndexOfChildMemberWithName(
                     name, omit_empty_base_classes, child_indexes))
               return child_indexes.size();
-            child_indexes.pop_back();
-
+            child_indexes = save_indices;
           } else if (field_name == name) {
             // We have to add on the number of base classes to this index!
             child_indexes.push_back(