Simplify <type_traits> implementations.

This patch rewrites a number of old meta-function implementations
that assumed const/volatile could not be safely applied to all types.
This is no longer the case, though for some types (Ex function types),
the const qualifier can be ignored.

The largest improvement in this patch is the reduction of is_function.
Thanks to Matt Calabrese for the improved implementation.

llvm-svn: 367749