[Parse] Add an attribute for typeEraser.

This will be used for compiler-driven type erasure for dynamic
replacement of functions with an opaque return type. For now, just
parse the attribute and ignore it.

include/swift/AST/Attr.h

@@ -1108,6 +1108,21 @@ public:
   }
 };
 
+/// The \c @_typeEraser(TypeEraserType) attribute.
+class TypeEraserAttr final : public DeclAttribute {
+  TypeLoc TypeEraserLoc;
+public:
+  TypeEraserAttr(SourceLoc atLoc, SourceRange range, TypeLoc typeEraserLoc)
+      : DeclAttribute(DAK_TypeEraser, atLoc, range, /*Implicit=*/false),
+        TypeEraserLoc(typeEraserLoc) {}
+
+  const TypeLoc &getTypeEraserLoc() const { return TypeEraserLoc; }
+
+  static bool classof(const DeclAttribute *DA) {
+    return DA->getKind() == DAK_TypeEraser;
+  }
+};
+
 /// Represents any sort of access control modifier.
 class AbstractAccessControlAttr : public DeclAttribute {
 protected: