Stubify broadly takes either tbd files or binary dylibs and turns them
into tbd files. In future patches, stubify will also allow additional
information to be embedded into the final TBD output too.

Add Util APIs to TextAPI for common operations used by readtapi for now. 

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.

> Add support for reading binary Mach-o dynamic libraries. It uses
libObject APIs for extracting information relevant to TAPI and tbd
files. This includes but is not limited to load commands encode data
like install names, current/compat versions, and symbols.

This originally broke because DylibReader uses Object and Object depends
on TextAPI. Breaking this up in a nested library prevents this cycle.

This reverts commit aa217eb04459039b5ff18b6b134020ed0248a241.
This reverts commit 634feddc84bfd402fc916d331627528c41346d8c.

This breaks buildbots by introducing cycle dependency between libObject
and TextAPI and breaks gcc compiles on buildbots.

* Reported in buildbot failures

Add support for reading binary Mach-o dynamic libraries. It uses
libObject APIs for extracting information relavant to TAPI and tbd
files. This includes but is not limited to load commands encode data
like install names, current/compat versions and symbols.

`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.

InterfaceFile is the in-memory representation for tbd files. Add APIs to
merge, extract, remove, and inline reexported libraries.

Reviewed By: zixuw

Differential Revision: https://reviews.llvm.org/D153398

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

Introduce initial reader for TBDv5 which is in JSON. This captures all
the currently understood fields within the internal structure
`InterfaceFile`.
New fields will be followed up in future PRs.

Reviewed By: pete

Differential Revision: https://reviews.llvm.org/D144156