More robust deployment target parsing on darwin

Summary:
This is a more robust way of figuring out implicit deployment target
from isysroot. It also handles iphone simulator target.

Reviewers: bob.wilson, t.p.northover

Reviewed By: t.p.northover

Subscribers: t.p.northover, cfe-commits

Differential Revision: http://reviews.llvm.org/D6939

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@226005 91177308-0d34-0410-b5e6-96231b3b80d8