@availability: implement serialization and AST printing

Swift SVN r16010

include/swift/AST/Attr.h

@@ -467,7 +467,7 @@ public:
     : DeclAttribute(DAK_asmname, AtLoc, Range, Implicit),
       Name(Name) {}
 
-  AsmnameAttr(StringRef Name)
+  AsmnameAttr(StringRef Name, bool Implicit)
     : AsmnameAttr(Name, SourceLoc(), SourceRange(), /*Implicit=*/true) {}
 
   /// The symbol name.

include/swift/Serialization/ModuleFormat.h

@@ -964,11 +964,22 @@ namespace decls_block {
 
   using AsmnameDeclAttrLayout = BCRecordLayout<
     Asmname_DECL_ATTR,
-    BCBlob // asmname
+    BCFixed<1>, // implicit flag
+    BCBlob      // asmname
+  >;
+
+  using AvailabilityDeclAttrLayout = BCRecordLayout<
+    Availability_DECL_ATTR,
+    BCFixed<1>, // implicit flag
+    BCFixed<1>, // is unconditionally unavailable?
+    BCVBR<5>,   // number of bytes in platform string
+    BCVBR<5>,   // number of bytes in message string
+    BCBlob      // platform, followed by message
   >;
 
   using ObjCDeclAttrLayout = BCRecordLayout<
     ObjC_DECL_ATTR,
+    BCFixed<1>, // implicit flag
     ObjCDeclAttrKindField, // kind
     BCArray<IdentifierIDField>
   >;