introduce a new TargetLibraryInfo pass, which transformations can use to

introduce a new TargetLibraryInfo pass, which transformations can use to
query about available library functions.  For now this just has
memset_pattern16, which exists on darwin, but it can be extended for a
bunch of other things in the future.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125965 91177308-0d34-0410-b5e6-96231b3b80d8