Parsing and type checking for the definition of isolated conformances

Allow a conformance to be "isolated", meaning that it stays in the same
isolation domain as the conforming type. Only allow this for
global-actor-isolated types.

When a conformance is isolated, a nonisolated requirement can be
witnessed by a declaration with the same global actor isolation as the
enclosing type.

include/swift/AST/ProtocolConformanceOptions.h

@@ -34,8 +34,8 @@ enum class ProtocolConformanceFlags {
   /// @retroactive conformance
   Retroactive = 0x08,
 
-  /// @safe conformance
-  Safe = 0x10,
+  /// @isolated conformance
+  Isolated = 0x10,
 
   // Note: whenever you add a bit here, update
   // NumProtocolConformanceOptions below.