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