[LeeLee26]

This makes the namespace-scope PidCache object constant-initialized instead of
dynamically initialized or constructed through a function-local static.

Previously, a dynamically initialized namespace-scope PidCache could be
affected by static initialization ordering:

TU1 static init
  -> get_cached_pid()
  -> PidCache::init()
  -> AtFork::registerHandler(this, ...)

TU2 static init, where Pid.cpp's namespace-scope PidCache is dynamically
initialized later
  -> PidCache state_ is zero-initialized / reset
  -> get_cached_pid()
  -> PidCache::init()
  -> AtFork::registerHandler(this, ...)
  -> runtime error: at-fork: append: duplicate

This version avoids that ordering by making the namespace-scope cache
constinit. The object is initialized during static initialization, before
any dynamic initialization runs, so there is no later dynamic initialization
step that can overwrite the state after registration.

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