[ManagedStatic] Reimplement double-checked locking with std::atomic.

This gets rid of the memory fence in the hot path (dereferencing the
ManagedStatic), trading for an extra mutex lock in the cold path (when
the ManagedStatic was uninitialized). Since this only happens on the
first accesses it shouldn't matter much. On strict architectures like
x86 this removes any atomic instructions from the hot path.

Also remove the tsan annotations, tsan knows how standard atomics work
so they should be unnecessary now.

llvm-svn: 274131

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