swift234

Reinstate watchOS and other SDK overlays to build against the public SDKs.

Un-revert the below commits with the following addition:
add declarations for posix_spawn related APIs to SwiftPrivateDarwinExtras.
posix_spawn-related APIs aren't available in the public SDKs, so force past
the availability by creating our own stubs in the internal DarwinExtras
library.

r31244, r31245

CMake: build all platforms except watchOS using the public SDK

Covers rdar://problem/21145996.
A step towards rdar://problem/21099318.

Switch SDK overlays to use the public SDK

I had to cut the dependency on CrashReporterClient.h and reimplement
some of that code inline in the Swift runtime.  This shoud be OK (even
though not very clean), since the layout of CrashReporter sections is
ABI.

rdar://21099318

Swift SVN r31252