[llvm] Add default empty header filter regex to root .clang-tidy (PR #167386)

[Baranov Victor via llvm-commits]

https://github.com/vbvictor created https://github.com/llvm/llvm-project/pull/167386

After https://github.com/llvm/llvm-project/pull/164165, we emit warnings from non-system headers by default.
This change only preserves functionality of `clang-tidy` as it was before the change.

>From e3515fb4574d8d92b2b8dffbfe716884e688fbfe Mon Sep 17 00:00:00 2001
From: Victor Baranov <bar.victor.2002 at gmail.com>
Date: Mon, 10 Nov 2025 23:45:34 +0300
Subject: [PATCH] Add default header filter regex to root .clang-tidy

---
 .clang-tidy | 1 +
 1 file changed, 1 insertion(+)

diff --git a/.clang-tidy b/.clang-tidy
index 06bb0f18e9d2e..2cda1b81de808 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -1,3 +1,4 @@
+HeaderFilterRegex: ''
 Checks: >
   -*,
   clang-diagnostic-*,