Re-introduce `copts` hacks for lib/AST includes.

Sadly, these are necessary AFAICT. There is a file `lib/AST/CXXABI.h`.
On case insensitive file systems like macOS this will collide with
`cxxabi.h` on the system if we use the `includes` trick to allow
file-relative `#include` of generated files.

I've tested this on both Linux and Windows to make sure it remains
reasonably portable.

Differential Revision: https://reviews.llvm.org/D112883