Move ASTRecordWriter into its own header; NFC.

Similar motivations to the movement of ASTRecordReader:
AbstractBasicWriter.h already has quite a few dependencies,
and it's going to get pretty large as we generate more and more
into it.  Meanwhile, most clients don't depend on this detail of
the implementation and shouldn't need to be recompiled.

I've also made OMPClauseWriter private, like it belongs.