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

This reverts commit 797b50d4.

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: https://reviews.llvm.org/D115544