[docs doxygen] Set STRIP_FROM_INC_PATH.

Without this option set, the documentation for a class in
e.g. `klang/AST/ASTNodes.h` will have doxygen say
`#include <ASTNodes.h>` on that page's documentation. With this set and
the appropriate directory specified, it now properly says
`#include <klang/AST/ASTNodes.h>`.

There's also the issue of having doxygen use "quoted" rather than
"angled" includes for this. E.g. print it as
`#include "klang/AST/ASTNodes.h"`
which is what the include lines for this file look like in our codebase.