[libcxx-commits] [libcxx] [libcxx] [modules] Fix relative paths with absolute LIBCXX_INSTALL_MODULES_DIR (PR #85756)

[Martin Storsjö via libcxx-commits]

mstorsjo wrote:

Argh, I see that this still is failing, if `CMAKE_INSTALL_PREFIX` is empty, with errors like this:
```
CMake Error at /b/s/w/ir/x/w/llvm-llvm-project/libcxx/modules/CMakeLists.txt:215 (file):
  file RELATIVE_PATH must be passed a full path to the directory:
  lib/x86_64-pc-windows-msvc
```

Let's revert it on main and restart with a new PR, as this will need a bit more modifications.

https://github.com/llvm/llvm-project/pull/85756