[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/Serialization.cpp

@@ -2108,6 +2108,7 @@ class Serializer::DeclSerializer : public DeclVisitor<DeclSerializer> {
     case DAK_RestatedObjCConformance:
     case DAK_ClangImporterSynthesizedType:
     case DAK_PrivateImport:
+    case DAK_TypeEraser:
       llvm_unreachable("cannot serialize attribute");
 
     case DAK_Count: