[MachO] Introduce MinVersion API.

While introducing support for MinVersionLoadCommand in llvm-readobj I noticed there's
no API to extract Major/Minor/Update components conveniently. Currently consumers
do the bit twiddling on their own, but this will change from now on.

I'll convert llvm-objdump (and llvm-readobj) in a later commit.

Differential Revision:	 http://reviews.llvm.org/D12282
Reviewed by:	rafael

llvm-svn: 245938