[llvm][TextAPI] adding inlining reexported libraries support

Summary:
[llvm][TextAPI] adding inlining reexported libraries support

* this patch adds reader/writer support for MachO tbd files.
The usecase is to represent reexported libraries in top level library
that won't need to exist for linker indirection because all of the
needed content will be inlined in the same document.

Reviewers: ributzka, steven_wu, jhenderson

Reviewed By: ributzka

Subscribers: JDevlieghere, hiraditya, mgrang, dexonsmith, rupprecht, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67646