Revert "Revert "Revert "Use `GNUInstallDirs` to support custom installation dirs. -- LLVM"""

This reverts commit 492de35df443d5f31480173d5f1274c7835cd3d8.

I tried to apply John's changes in 8d897ec91528 that were expected to
fix his patch but that didn't work unfortunately.

Reverting this again to fix the macOS bots and leave him more time to
investigate the issue.

GitOrigin-RevId: 30fc88bf1dc15a72e2d9809d28019d386b7a7cc0
1 file changed