Implement generalized table lookups for upper, lower, and character

traits.

To the best of my knowledge, this will not break the ABI for Apple.
However, it does introduce three publicly visible (although with
reserved name) functions that will fail to link against the just-shipped
Apple version of libc++. Since they are not used in any inline
functions, no actual breakage should occur.

If Howard doesn't want to put undefined functions (even internal ones)
into a header, they could be surrounded by additional conditional
compilation.

llvm-svn: 134781