This replaces swiftMSVCRT with swiftCRT. The big difference here is
that the `visualc` module is no longer imported nor exported. The
`visualc` module remains in use for a singular test wrt availability,
but this should effectively remove the need for the `visualc` module.
The difference between the MSVCRT and ucrt module was not well
understood by most. MSVCRT provided ucrt AND visualc, combining pieces
of the old MSVCRT and the newer ucrt. The ucrt module is what you
really wanted most of the time, however, would need to use MSVCRT for
the convenience aliases for type-generic math and the deprecated math
constants.
Unfortunately, we cannot shadow the `ucrt` module and create a Swift SDK
overlay for ucrt as that seems to result in circular dependencies when
processing the `_Concurrency` module.
Although this makes using the C library easier for most people, it has a
more important subtle change: it cleaves the dependency on visualc.
This means that this enables use of Swift without Visual Studio for the
singular purpose of providing 3 header files. Additionally, it removes
the need for the installation of 2 of the 4 support files. This greatly
simplifies the deployment process on Windows.
There are things about this that I'm far from sold on. In
particular, I'm concerned that in order to implement escalation
correctly, we're going to have to add a status record for the
fact that the task is being executed, which means we're going
to have to potentially wait to acquire the status lock; overall,
that means making an extra runtime function call and doing some
atomics whenever we resume or suspend a task, which is an
uncomfortable amount of overhead.
The testing here is pretty grossly inadequate, but I wanted to
lay down the groundwork here.
This effectively reverts #31183 -- we need to match the install name convention of the other stdlib libraries.
From the review feedback:
> The right way to load the stdlib & runtime libraries from a custom toolchain is to set `DYLD_LIBRARY_PATH` when executing the generated binary. This is how we run tests against the just-built libraries and this is how downloadable toolchain snapshots are currently configured in Xcode -- see #33178
`get_async_continuation[_addr]` begins a suspend operation by accessing the continuation value that can resume
the task, which can then be used in a callback or event handler before executing `await_async_continuation` to
suspend the task.
When an actor class has its `enqueue(partialTask:)` implicitly
synthesized, also synthesize a stored property for the actor's queue.
The type of the property is defined by the _Concurrency library
(`_DefaultActorQueue`), and it will be initialized with a call to
`_defaultActorQueueCreate` (also provided by the _Concurrency
library).
Also synthesize the body of the implicitly-generated
`enqueue(partialTask:)`, which will be a call to
`_defaultActorQueueEnqueuePartialTask(actor:queue:partialTask:)`.
Together, all of these allow us to experiment with the form of the
queue and the queue operation without affecting the type checker.
When `enqueue(partialTask:)` is not implicitly synthesized, the queue
storage is not synthesized either. In such cases, the user has taken
over the execution of tasks for the actor, rather than using the
default implementation.
Introduce a new Actor protocol, which is a class-bound protocol with only
one requirement:
func enqueue(partialTask: PartialAsyncTask)
All actor classes implicitly conform to this protocol, and will synthesize
a (currently empty) definition of `enqueue(partialTask:)` unless a suitable
one is provided explicitly.
The experimental concurrency model will require a supporting runtime
and possibly end-user-visible library constructs. Introduce a stub of
such a library, enabled by a new `build-script` option
`--enable-experimental-concurrency`, so we have a place to put this
work.
