ObjCInterfaceRecords roughly align to the objc-classes key in tbd-files.
They condensely represent up to 3 symbols. The problem here is that when
represented this way, we lose granularity when these symbols could have
different linkages or outright don't exist. This can happen frequently
in interoptable code generated by the swift compiler. This adds fields
and utility functions to express unique properties for these symbols. If
the record does represent the same properties across all of its symbols,
it will be treated the same in the TBD. Otherwise it will be printed in
global's section.

Reviewed seperately before by Juergen Ributzka

A distinction that doesn't _usually_ matter is that the
MachO::SymbolKind is really a mapping of entries in TBD files not
symbols. To better understand this, rename the enum so it represents an
encoding mapped to TBDs as opposed to symbols alone.

For example, it can be a bit confusing that "GlobalSymbol" is a enum
value when all of those values can represent a GlobalSymbol.

SymbolSet is a structure that acts as a simple container class for exported symbols that
belong to a library interface. It allows tapi to decouple the globals
from the other library attributes. It's uniqued by symbol name and `kind`, which all contain their assigned target triples.

Reviewed By: zixuw

Differential Revision: https://reviews.llvm.org/D149860