AsmPrinter: Fix wrong OS X versions being emitted for darwin triples

The version numbers of the darwin kernel are different from the version
numbers of OS X, so we need adjustments if we had "*-*-darwin" triples.
Use the existing utility functions in TargetTriple for this.

Fixes rdar://22056966

Differential Revision: http://reviews.llvm.org/D14601

llvm-svn: 257555