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
(cherry picked from commit 40b41ca3d8fb69f5bec2ce65f0cb7490c3b048a6)

Bug: http://b/189279320
Test: treehugger
Change-Id: I7b0d0db251a3ac960349321daca3fbc97f673b5b
1 file changed