[clang][cmake] Use `GNUInstallDirs` to support custom installation dirs

I am breaking apart D99484 so the cause of build failures is easier to
understand.

Differential Revision: https://reviews.llvm.org/D117419

NOKEYCHECK=True
GitOrigin-RevId: 10d0d8c0c1db57b7ff465df7ced78a42a20d592d
1 file changed