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

This reverts commit 797b50d4be873b4662983413a06806fca544c276.

See the original D99484. @mib who noticed the original problem could not longer
reproduce it, after I tried and also failed. We are threfore hoping it went
away on its own!

Reviewed By: mib

Differential Revision:

GitOrigin-RevId: 492de35df443d5f31480173d5f1274c7835cd3d8
1 file changed