Add a utility to clone an @available attribute.

Author: rjmccall

include/swift/AST/Attr.h

@@ -743,6 +743,8 @@ class AvailableAttr : public DeclAttribute {
                          llvm::VersionTuple Obsoleted
                          = llvm::VersionTuple());
 
+  AvailableAttr *clone(ASTContext &C, bool implicit) const;
+
   static bool classof(const DeclAttribute *DA) {
     return DA->getKind() == DAK_Available;
   }

lib/AST/Attr.cpp

@@ -1480,6 +1480,20 @@ bool AvailableAttr::isActivePlatform(const ASTContext &ctx) const {
   return isPlatformActive(Platform, ctx.LangOpts);
 }
 
+AvailableAttr *AvailableAttr::clone(ASTContext &C, bool implicit) const {
+  return new (C) AvailableAttr(implicit ? SourceLoc() : AtLoc,
+                               implicit ? SourceRange() : getRange(),
+                               Platform, Message, Rename,
+                               Introduced ? *Introduced : llvm::VersionTuple(),
+                               implicit ? SourceRange() : IntroducedRange,
+                               Deprecated ? *Deprecated : llvm::VersionTuple(),
+                               implicit ? SourceRange() : DeprecatedRange,
+                               Obsoleted ? *Obsoleted : llvm::VersionTuple(),
+                               implicit ? SourceRange() : ObsoletedRange,
+                               PlatformAgnostic,
+                               implicit);
+}
+
 Optional<OriginallyDefinedInAttr::ActiveVersion>
 OriginallyDefinedInAttr::isActivePlatform(const ASTContext &ctx) const {
   OriginallyDefinedInAttr::ActiveVersion Result;