[OPENMP] Move OMPClauseReader/Writer classes to ASTReader/Writer (NFC)

Move declarations for OMPClauseReader, OMPClauseWriter to ASTReader.h 
and ASTWriter.h and move implementation to ASTReader.cpp and 
ASTWriter.cpp. This change helps generalize the serialization of
OpenMP clauses and will be used in the future implementation of new 
OpenMP directives (e.g. requires).

Patch by Patrick Lyster

Differential Revision: https://reviews.llvm.org/D52097

llvm-svn: 342322