Add experimental flag to enable @unsafe in Volatile library

Author: DougGregor

stdlib/public/Volatile/CMakeLists.txt

@@ -42,7 +42,7 @@ if(SWIFT_SHOULD_BUILD_EMBEDDED_STDLIB)
       Volatile.swift
 
       SWIFT_COMPILE_FLAGS
-        -Xcc -D__MACH__ -Xcc -D__APPLE__ -Xcc -ffreestanding -enable-experimental-feature Embedded
+        -Xcc -D__MACH__ -Xcc -D__APPLE__ -Xcc -ffreestanding -enable-experimental-feature Embedded -enable-experimental-feature AllowUnsafeAttribute
         -parse-stdlib
       C_COMPILE_FLAGS
         -D__MACH__ -D__APPLE__ -ffreestanding