Add default empty header filter regex to root .clang-tidy (#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.

.clang-tidy

@@ -1,3 +1,4 @@
+HeaderFilterRegex: ''
 Checks: >
   -*,
   clang-diagnostic-*,