[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.

lib/Serialization/ModuleFormat.h

@@ -1823,6 +1823,12 @@ namespace decls_block {
     BCArray<IdentifierIDField>
   >;
 
+  using TypeEraserDeclAttrLayout = BCRecordLayout<
+    TypeEraser_DECL_ATTR,
+    BCFixed<1>, // implicit flag
+    TypeIDField // type eraser type
+  >;
+
   using CustomDeclAttrLayout = BCRecordLayout<
     Custom_DECL_ATTR,
     BCFixed<1>,  // implicit flag