Store SourceManager pointer on PrintingPolicy in the case where we're dumping,

and remove ASTContext reference (which was frequently bound to a dereferenced
null pointer) from the recursive lump of printPretty functions. In so doing,
fix (at least) one case where we intended to use the 'dump' mode, but that
failed because a null ASTContext reference had been passed in.

llvm-svn: 162011