Refactor Symbol visitor code.

Type visitor code had already been refactored previously to
decouple the visitor and the visitor callback interface.  This
was necessary for having the flexibility to visit in different
ways (for example, dumping to yaml, reading from yaml, dumping
to ScopedPrinter, etc).

This patch merely implements the same visitation pattern for
symbol records that has already been implemented for type records.

llvm-svn: 283609