[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.def

@@ -519,6 +519,11 @@ SIMPLE_DECL_ATTR(_inheritsConvenienceInitializers,
   APIStableToAdd | ABIStableToAdd | APIBreakingToRemove | ABIBreakingToRemove,
   93)
 
+DECL_ATTR(_typeEraser, TypeEraser,
+  OnProtocol | UserInaccessible | NotSerialized |
+  ABIStableToAdd | ABIBreakingToRemove | APIStableToAdd | APIBreakingToRemove,
+  94)
+
 SIMPLE_DECL_ATTR(IBSegueAction, IBSegueAction,
   OnFunc |
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove,

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:

include/swift/AST/DiagnosticsParse.def

@@ -1488,6 +1488,11 @@ ERROR(attr_dynamic_replacement_expected_for,none,
 ERROR(attr_dynamic_replacement_expected_colon,none,
       "expected ':' after @_dynamicReplacement(for", ())
 
+ERROR(attr_type_eraser_expected_type_name,none,
+      "expected a type name in @_typeEraser()", ())
+ERROR(attr_type_eraser_expected_rparen,none,
+      "expected ')' after type name for @_typeEraser", ())
+
 ERROR(attr_private_import_expected_rparen,none,
       "expected ')' after function name for @_private", ())
 ERROR(attr_private_import_expected_sourcefile, none,