Always use is_initialized and set_initialized in cxa_guard.cpp

This patch is part of a series of cleanups to cxa_guard.cpp.
It should have no functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@357668 91177308-0d34-0410-b5e6-96231b3b80d8