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

This reverts commit r256642 and restores r256620 now that Tobias has
updated Polly.

There are still some potential problems with the code in Polly that I've
sent post-commit review about, but they're unlikely to break anything in
practice, and I'd like to avoid the rest of LLVM and Clang regressing
here.

llvm-svn: 256656