[clang-tools-extra] [clang-tidy] Add bugprone-custom-errno-declaration check (PR #213015)

[Zeyi Xu via cfe-commits]

================
@@ -0,0 +1,25 @@
+// RUN: %check_clang_tidy %s bugprone-custom-errno-declaration %t
+
+namespace cerrno_test_0 {
+    extern int errno;
----------------
zeyi2 wrote:

Could we add tests for:

```cpp
extern int errno, preserved;
```

https://github.com/llvm/llvm-project/pull/213015