[libc++] Improve CMake include directory search.

This patch has been tested in D70631, but it should be reviewed
separately.

Reviewed By: ldionne, #libc

Differential Revision: https://reviews.llvm.org/D114248

GitOrigin-RevId: cc9fdedba7c9b19fed24a7b67f63a3a2e7a729bd
1 file changed