Add PLATFORM constants for iOS, tvOS, and watchOS simulators

Summary:
Add PLATFORM constants for iOS, tvOS, and watchOS simulators, as well
as human readable names for these constants, to the Mach-O file format
header files. 

rdar://46854119

Reviewers: ab, davide

Reviewed By: ab, davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D55905

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349779 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed