[clang] [Clang][Driver] Support relative paths for CLANG_CONFIG_FILE_SYSTEM_DIR (PR #110962)

[Carlo Cabrera via cfe-commits]

carlocab wrote:

I authored my commit using a working email address. See the [header](https://github.com/llvm/llvm-project/commit/e31545f666eb4ca32030956a38dbc4078a64068c.patch) of the patch:
```
>From e31545f666eb4ca32030956a38dbc4078a64068c Mon Sep 17 00:00:00 2001
From: Carlo Cabrera <github at carlo.cab>
Date: Thu, 3 Oct 2024 14:15:07 +0800
Subject: [PATCH] [Clang][Driver] Support relative paths for
 CLANG_CONFIG_FILE_SYSTEM_DIR
```
Is that insufficient?

https://github.com/llvm/llvm-project/pull/110962