[llvm-pdbdump] Rewrite dumper using visitor pattern.

[llvm-pdbdump] Rewrite dumper using visitor pattern.

This increases the flexibility of how to dump different
symbol types -- necessary for context-sensitive formatting of
symbol types -- and also improves the modularity by allowing
the dumping to be implemented in the actual dumper, as opposed
to in the PDB library.

llvm-svn: 230184