When late parsed templates are used with PCH, tokens are serialized. The
existing code does not handle annotation tokens which can occur due to
This patch implements the serialization for annot_pragma_loop_hint.
This also enables use of OpenMP pragmas and #pragma unused which do not
need special serialization of the PtrData field.
There are other pragmas that can be used with late template parsing which
we can implement in separately patches as needed.