Further refactor cxa_guard.cpp

This patch is a part of a series of patches to cleanup
our implementation of __cxa_acquire et al. No functionality
change was intended.

This patch does two primary things.

It introduces the GuardObject class to abstract the reading
and writing to the guard object. In future, it will be used
to ensure atomic accesses are used when needed.

It also introduces the GuardValue class used to represent
values of the guard object. It is an abstraction to access
and write to the various different bits of a guard.

llvm-svn: 357804