tsan: rearrange thread state callbacks (NFC)

Thread state functions are split into 2 parts:
tsan entry function (e.g. ThreadStart) and thread registry
state change callback (e.g. OnStart). Currently these
pairs of functions are located far from each other and
in reverse order. This makes it hard to read and follow the logic.
Reorder the code so that OnFoo directly follows ThreadFoo.
No other code changes.

Reviewed By: melver

Differential Revision: https://reviews.llvm.org/D110132