[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230184 91177308-0d34-0410-b5e6-96231b3b80d8