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 absolute path gets prepended with `CMAKE_INSTALL_PREFIX`. Call the adjusted variable `outpath_abs` instead of `binpath` since it isn't always an executable being symlinked into place. Also fix one stray `bin` -> `CMAKE_INSTALL_LIBDIR`

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 with `CMAKE_INSTALL_PREFIX`. 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.Call the adjusted variable `outpath_abs` instead of `binpath` since it isn't always an executable being symlinked into place. Also fix one stray `bin` -> `CMAKE_INSTALL_LIBDIR`