Parser/Sema/SILGen changes for @_dynamicReplacement(for:)

Dynamic replacements are currently written in extensions as

extension ExtendedType {
  @_dynamicReplacement(for: replacedFun())
  func replacement() { }
}

The runtime implementation allows an implementation in the future where
dynamic replacements are gather in a scope and can be dynamically
enabled and disabled.

For example:

dynamic_extension_scope CollectionOfReplacements {
  extension ExtentedType {
    func replacedFun() {}
  }

  extension ExtentedType2 {
    func replacedFun() {}
  }
}

CollectionOfReplacements.enable()
CollectionOfReplacements.disable()

include/swift/SIL/SILDeclRef.h

@@ -252,6 +252,12 @@ struct SILDeclRef {
     return kind == Kind::IVarInitializer || kind == Kind::IVarDestroyer;
   }
 
+  /// True if the SILDeclRef references an allocating or deallocating entry
+  /// point.
+  bool isInitializerOrDestroyer() const {
+    return kind == Kind::Initializer || kind == Kind::Destroyer;
+  }
+
   /// \brief True if the function should be treated as transparent.
   bool isTransparent() const;
   /// \brief True if the function should have its body serialized.