<rdar://problem/13615607> Include SDK version information in the module hash.

This is a Darwin-SDK-specific hash criteria used to identify a
particular SDK without having to hash the contents of all of its
headers. If other platforms have such versioned files, we should add
those checks here.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@179346 91177308-0d34-0410-b5e6-96231b3b80d8