[Clang] Correct when Itanium ABI guard variables are set for non-block...

[Clang] Correct when Itanium ABI guard variables are set for non-block variables with static or thread storage duration.

Previously, Itanium ABI guard variables were set after initialization was
complete for non-block declared variables with static and thread storage
duration. That resulted in initialization of such variables being restarted
in cases where the variable was referenced while it was still under
construction. Per C++20 [class.cdtor]p2, such references are permitted
(though the value obtained by such an access is unspecified). The late
initialization resulted in recursive reinitialization loops for cases like
this:
  template<typename T>
  struct ct {
    struct mc {
      mc() { ct<T>::smf(); }
      void mf() const {}
    };
    thread_local static mc tlsdm;
    static void smf() { tlsdm.mf(); }
  };
  template<typename T>
  thread_local typename ct<T>::mc ct<T>::tlsdm;
  int main() {
    ct<int>::smf();
  }

With this change, guard variables are set before initialization is started
so as to avoid such reinitialization loops.

Fixes https://github.com/llvm/llvm-project/issues/57828

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D135919