Defined SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY to describe
whether the standard library will use the task-to-thread model for
concurrency.  It is true only for freestanding non-Darwin stdlibs.

When it is true, SWIFT_CONCURRENCY_TASK_TO_THREAD_MODEL is defined
during stdlib compilation of both Swift and C++ sources.

Added an option to LangOptions to specify which concurrency model is
used, either standard or task-to-thread.  When
SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY is true, the model is
specified to be task-to-thread.

In this mode, the following are disabled:

- task creation
- global actors
- MainActor
- custom executors

Previously getAsyncClosureEntryPointAndContextSize took both a pointer
for a function and a closure context.  That was a relic of a temporary
ABI where async function pointers were not used in all cases.  Now that
that ABI is long gone, async function pointers are always used.
Consequently, the closure context argument was unused.  Here, the
closure context argument is removed.

So that this functionality is available elsewhere, extract it from
swift_task_create_commonImpl into a reusable static functions.

The new intrinsic, exposed via static functions on Task<T, Never> and
Task<T, Error> (rethrowing), begins an asynchronous context within a
synchronous caller's context.  This is only available for use under the
task-to-thread concurrency model, and even then only under SPI.