[BinaryFormat] Teach identify_magic about Tapi files.

Summary:
Tapi files are YAML files that start with the !tapi tag. The only execption are
TBD v1 files, which don't have a tag. In that case we have to scan a little
further and check if the first key "archs" exists.

This is the first patch in a series of patches to add libObject support for
text-based dynamic library (.tbd) files.

This patch is practically exactly the same as D37820, that was never pushed to master,
and is needed for future commits related to reading tbd files for llvm-nm

Reviewers: ributzka, steven_wu, bollu, espindola, jfb, shafik, jdoerfert

Reviewed By: steven_wu

Subscribers: dexonsmith, llvm-commits

Tags: #llvm, #clang, #sanitizers, #lldb, #libc, #openmp

Differential Revision: https://reviews.llvm.org/D66149

llvm-svn: 369579