If `CMAKE_INSTALL_BINDIR` is a different absolute path per project, as it is with NixOS when we install every package to its own prefix, the old way fails when the downstream packages try to install symlinks in the now-immutable `LLVM_TOOLS_INSTALL_DIR`. The new way ensures the downstream projects' prefixes will be used instead, as desired.

If `CMAKE_INSTALL_BINDIR` is a different absolute path per project, as it is with NixOS when we install every package to its own prefix, the old way fails when the absolute path gets prepended. There are still some issues with dowstream packages using `LLVM_TOOLS_INSTALL_DIR` which also may be absolute and just for LLVM proper, but that will be addressed in a future commit.

If `CMAKE_INSTALL_BINDIR` is a different absolute path per project, as it is with NixOS when we install every package to its own prefix, the old way fails when the downstream packages try to install symlinks in the now-immutable `LLVM_TOOLS_INSTALL_DIR`absolute path gets prepended. The new way ensures the downstream projects' prefixes will be used insteadre are still some issues with dowstream packages using `LLVM_TOOLS_INSTALL_DIR` which also may be absolute and just for LLVM proper, as desiredbut that will be addressed in a future commit.