clang-extra: fix incorrect use of std::lock_guard by adding variable name (identified by MSVC [[nodiscard]] error)
std::lock_guard is an RAII class that needs a variable name whose scope determines the guard's lifetime. This particular usage lacked a variable name, meaning the guard could be destroyed before the line that it was indented to protect.
This line was identified by building clang with the latest MSVC preview release, which declares the std::lock_guard constructor to be [[nodiscard]] to draw attention to such issues.
Reviewed By: kadircet
Differential Revision: https://reviews.llvm.org/D95725