[Concurrency] Introduce runtime detection of data races.

Through various means, it is possible for a synchronous actor-isolated
function to escape to another concurrency domain and be called from
outside the actor. The problem existed previously, but has become far
easier to trigger now that `@escaping` closures and local functions
can be actor-isolated.

Introduce runtime detection of such data races, where a synchronous
actor-isolated function ends up being called from the wrong executor.
Do this by emitting an executor check in actor-isolated synchronous
functions, where we query the executor in thread-local storage and
ensure that it is what we expect. If it isn't, the runtime complains.
The runtime's complaints can be controlled with the environment
variable `SWIFT_UNEXPECTED_EXECUTOR_LOG_LEVEL`:

  0 - disable checking
  1 - warn when a data race is detected
  2 - error and abort when a data race is detected

At an implementation level, this introduces a new concurrency runtime
entry point `_checkExpectedExecutor` that checks the given executor
(on which the function should always have been called) against the
executor on which is called (which is in thread-local storage). There
is a special carve-out here for `@MainActor` code, where we check
against the OS's notion of "main thread" as well, so that `@MainActor`
code can be called via (e.g.) the Dispatch library's
`DispatchQueue.main.async`.

The new SIL instruction `extract_executor` performs the lowering of an
actor down to its executor, which is implicit in the `hop_to_executor`
instruction. Extend the LowerHopToExecutor pass to perform said
lowering.

lib/Serialization/SerializeSIL.cpp

@@ -1381,6 +1381,7 @@ void SILSerializer::writeSILInstruction(const SILInstruction &SI) {
   case SILInstructionKind::BeginCOWMutationInst:
   case SILInstructionKind::EndCOWMutationInst:
   case SILInstructionKind::HopToExecutorInst:
+  case SILInstructionKind::ExtractExecutorInst:
   case SILInstructionKind::AbortApplyInst:
   case SILInstructionKind::EndApplyInst:
   case SILInstructionKind::ReturnInst: