This is relanding of https://github.com/llvm/llvm-project/pull/69253. `TestTemplatePackArgs.py` is passing now. https://github.com/llvm/llvm-project/pull/68012/files added new data formatters for LibStdC++ std::variant. However, this formatter can crash if std::variant's index field has invalid value (exceeds the number of template arguments). This can happen if the current IP stops at a place std::variant is not initialized yet. This patch fixes the crash by ensuring the index is a valid value and fix GetNthTemplateArgument() to make sure it is not crashing. Co-authored-by: jeffreytan81 <jeffreytan@fb.com>