Move ASTRecordReader into its own header; NFC.

Move ASTRecordReader into its own header; NFC.

AbstractBasicReader.h has quite a few dependencies already,
and that's only likely to increase.  Meanwhile, ASTRecordReader
is really an implementation detail of the ASTReader that is only
used in a small number of places.

I've kept it in a public header for the use of projects like Swift
that might want to plug in to Clang's serialization framework.

I've also moved OMPClauseReader into an implementation file,
although it can't be made private because of friendship.