[ptr-traits] Implement the base pointer traits using the actual

alignment of the pointee type!

This is the culmination of the ptr-traits work. Now the compiler will
catch me if I try to use a pointer to an empty struct as a key in
a dense map or inside a PointerIntPair or PointerUnion! This is much,
much better than sometimes corrupting data (and other times working
fine) due to insufficient alignment.

It also means that we will be much more diligent about rejecting other
uses of these constructs that aren't safe.

It also means that we can now be more aggressive with the constructs
when we actually have guaranteed higher alignment without specializing
stuff. I'll be going through and cleaning up all the current overrides
of these traits which are no longer necessary.

Many thanks to Richard, David, and others who helped me get all of this
together.

llvm-svn: 256620