[codeview] Refactor symbol records to use same pattern as types.

This will pave the way to introduce a full fledged symbol visitor
similar to how we have a type visitor, thus allowing the same
dumping code to be used in llvm-readobj and llvm-pdbdump.

Differential Revision: http://reviews.llvm.org/D20384
Reviewed By: rnk

llvm-svn: 270475