The `try await` ordering is both easier to read and indicates the order
of operations better, because the suspension point occurs first and
then one can observe a thrown error.
We haven't implemented any support for top-level async code yet, so
reject it in effects checking rather than crashing in the SIL verifier.
Fixes rdar://71512818.
The initializer of an 'async let' is executed as a separate child task
that will run concurrently with the main body of the function. Model
the semantics of this operation by wrapping the initializer in an
async, escaping autoclosure (representing the evaluation of the child
task), and then a call to that autoclosure (to
This is useful both for actor isolation checking, which needs to treat
the initializer as executing in concurrent code, and also (eventually)
for code generation, which needs to have that code in a closure so
that it can be passed off to the task-creation functions.
There are a number of issues with this implementation producing
extraneous diagnostics due to this closure transformation, which will
be addressed in a follow-up commit.
Extend effects checking to ensure that each reference to a variable
bound by an 'async let' is covered by an "await" expression and occurs
in a suitable context.
