Add LC_BUILD_VERSION load command

Summary:
Add a new load command LC_BUILD_VERSION. It is a generic version of
LC_*_VERSION_MIN load_command used on Apple platforms. Instead of having
a seperate load command for each platform, LC_BUILD_VERSION is recording
platform info as an enum. It also records SDK version, min_os, and tools
that used to build the binary.

rdar://problem/29781291

Reviewers: enderby

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29044

llvm-svn: 292824