AST: Introduce a Swift runtime availability domain.

Add support for the `Swift` availability domain, which represents availability
with respect to the Swift runtime. Use of this domain is restricted by the
experimental feature `SwiftRuntimeAvailability`.

lib/Serialization/ModuleFormat.h

@@ -776,6 +776,7 @@ enum class AvailabilityDomainKind : uint8_t {
   Embedded,
   Platform,
   Custom,
+  SwiftRuntime,
 };
 using AvailabilityDomainKindField = BCFixed<3>;