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

llvm-svn: 226005