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

llvm-svn: 349779