[Concurrency] Improve data-race detection for locals and globals

Improve data-race detection for local variables and global/static variables
* Only diagnose these cases at all, not (e.g.) class instance members
* For local functions, they run concurrently if they were referenced somewhere
  that runs concurrently

The latter required moving checking of both actor isolation and effects
for local functions to the point at which their enclosing (non-local)
functions are checked, because we need to reason about the
type-checked bodies of local functions.

Author: DougGregor