To achieve this, add a new cache variable
`SWIFT_EMBEDDED_STDLIB_SDKS_FOR_TARGET_TRIPLES` to be set like in the
following examples.
```
-DSWIFT_EMBEDDED_STDLIB_SDKS_FOR_TARGET_TRIPLES=aarch64-vendor-os@/usr/local/aarch64-vendor-os-sdk;aaarch-vendor-anotheros@/opt/aarch64-vendor-anotheros-sdk
```
We chose to use another setting instead of e.g. folding this into
`SWIFT_EMBEDDED_STDLIB_EXTRA_TARGET_TRIPLES` so it is clear this is opt
in and it does not regress existing configurations that set the SDK
directly (like it's the case for the WASM stdlib).
Addresses rdar://162368529
If you use SwiftStdlibCurrentOS availability, you will be able to
use new types and functions from within the implementation. This
works by, when appropriate, building with the CurrentOS availability
set to the current deployment target.
rdar://150944675
Annotate all of the `Unsafe*` types and `unsafe` functions in the standard
library (including concurrency, synchronization, etc.) as `@unsafe`. Add a
few tests to ensure that we detect uses of these types in clients that
have disabled unsafe code.
