[clang-tools-extra] [clang-tidy] Add misc-shadowed-namespace-function check (PR #168406)

[Baranov Victor via cfe-commits]

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@@ -0,0 +1,232 @@
+// RUN: %check_clang_tidy %s misc-shadowed-namespace-function %t
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vbvictor wrote:

```suggestion
// RUN: %check_clang_tidy %s -std=c++98-or-later misc-shadowed-namespace-function %t
```

https://github.com/llvm/llvm-project/pull/168406