AST: Make the legacy AvailableAttr constructor private.

Author: tshortli

include/swift/AST/Attr.h

@@ -756,7 +756,7 @@ class AvailableAttr : public DeclAttribute {
     NoAsync,
   };
 
-public:
+private:
   AvailableAttr(SourceLoc AtLoc, SourceRange Range, PlatformKind Platform,
                 StringRef Message, StringRef Rename,
                 const llvm::VersionTuple &Introduced,
@@ -767,6 +767,7 @@ class AvailableAttr : public DeclAttribute {
                 PlatformAgnosticAvailabilityKind PlatformAgnostic,
                 bool Implicit, bool IsSPI, bool IsForEmbedded = false);
 
+public:
   AvailableAttr(SourceLoc AtLoc, SourceRange Range,
                 const AvailabilityDomain &Domain, Kind Kind, StringRef Message,
                 StringRef Rename, const llvm::VersionTuple &Introduced,