Upstream support for macCatalyst Mach-O binaries.

On macOS one Mach-O slice can contain multiple load commands: One load
command for being loaded into a macOS process and one load command for
being loaded into a macCatalyst process. This patch adds support for
the new load command and makes sure ObjectFileMachO returns the
Architecture that matches the Module.

Differential Revision: https://reviews.llvm.org/D66626

llvm-svn: 369814