[compiler-rt][builtins] Add __isOSVersionAtLeast()

This predicate compares the host's marketing OS version to one passed as
argument. Currently, only darwin targets are supported. This is done by parsing
the SystemVersion.plist file.

Also added in this patch is some lit testing infrastructure for builtins, which
previously had none. This part of the patch was written by Alex Lorenz (with
some minor modifications).

This patch is part of a feature I proposed here:
http://lists.llvm.org/pipermail/cfe-dev/2016-July/049851.html

Differential revision: https://reviews.llvm.org/D30136

llvm-svn: 297382