[TextAPI] Arch&Platform to Target

Summary:
This is a patch for updating TextAPI/Macho to read in targets as opposed to arch/platform.
This is because in previous versions tbd files only supported a single platform but that is no longer the case,
so, now its tracked by unique triples.
This precedes a seperate patch that will add  the TBD-v4 format

Reviewers: ributzka, steven_wu, plotfi, compnerd, smeenai

Reviewed By: ributzka

Subscribers: mgorny, hiraditya, dexonsmith, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D67527

llvm-svn: 372396