SIOF-proof thread local

SIOF-proof thread local

Summary: Right now ThreadLocal & friends don't operate correctly when used as a
static variable (which is the idiomatic way to use them). The TLS id is
allocated in the static constructor so anybody who uses the ID prior to
first use would use an invalid ID. This makes ThreadLocal unusable for core
code such as per-thread reference counting.

This diff allocates the ID on first use. By making the invalid ID maxint we
avoid adding any extra branches in the fast path. We can then make the
constructor a constexpr meaning that initialization will happen prior to
any code running.

Reviewed By: @meyering

Differential Revision: D2457989

fb-gh-sync-id: 21d0c0d00c638fbbd36148d14d4c891f66f83706