Support -miphoneos-version-min in clang-cc.

 - Patch by Shantonu Sen (with a minor tweak to split out
   getDarwin{OSX,IPhoneOS}Defines)!

 - <rdar://problem/6776277> Need clang-cc/ccc-analyzer support for
   -miphoneos-version-min

llvm-svn: 68815