Introduce RecordVisitor. This is used for different clients that want to
extract information out of RecordSlice types.
The first and immediate use case is for serializing symbol information
into TBD files.
`Record`'s hold target triple specific information about APIs and
symbols. This holds information about the relationship between ObjC
symbols and their linkage properties. It will be used to compare and run
significant operations between the frontend representation of symbols in
AST and symbol information extracted from Mach-O binaries. This differs
from the lighter weight Symbol and SymbolSet class where they are
deduplicated across targets and only represent exported symbols, that
class is mostly used for serializing.
