Some notes:
1. This is not actually wired up to any part of codegen. Instead, this PR just
has the code necessary to parse the attribute and to ensure that we use it only
on local lets. The rest will come in subsequent commits.
2. I am allowing for the attribute to be attached to generic things in Sema
since we do not have enough information in the TypeChecker to distinguish in
between structs with a type parameter but that have all non-generic stored vars
from one with generic stored vars. We can only support the later with opaque
values but the former we can support without opaque values (and is one of the
use cases we are interested in).
rdar://83957088
This attribute creates an unavailable extension with a `Sendable` conformance so that the type is explicity marked as not being `Sendable`.
We also fully suppress diagnostics about unavailable Sendable conformances in Swift 5 mode code. (This is not fully developed yet—it should return to being a warning in concurrent contexts.)
The behavior when a @_nonSendable and a Sendable conformance are both on the same type is also not right yet.
