[AST][Sema] Add ValueDecl::hasCurriedSelf

This is a minimal cherry-pick of swiftlang#25170.

Author: hamishknight

include/swift/AST/Decl.h

@@ -2697,7 +2697,11 @@ class ValueDecl : public Decl {
   /// True if this is a C function that was imported as a member of a type in
   /// Swift.
   bool isImportAsMember() const;
-  
+
+  /// Returns true if the declaration's interface type is a function type with a
+  /// curried self parameter.
+  bool hasCurriedSelf() const;
+
   /// Get the decl for this value's opaque result type, if it has one.
   OpaqueTypeDecl *getOpaqueResultTypeDecl() const;
 
@@ -7168,6 +7172,14 @@ inline bool ValueDecl::isImportAsMember() const {
   return false;
 }
 
+inline bool ValueDecl::hasCurriedSelf() const {
+  if (auto *afd = dyn_cast<AbstractFunctionDecl>(this))
+    return afd->hasImplicitSelfDecl();
+  if (isa<EnumElementDecl>(this))
+    return true;
+  return false;
+}
+
 inline bool Decl::isPotentiallyOverridable() const {
   if (isa<VarDecl>(this) ||
       isa<SubscriptDecl>(this) ||