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

[Denis Mikhailov via cfe-commits]

denzor200 wrote:

> It would be nice to add some tests with nested namespace (`namespace A::B {}`) and inline namespace.

We have already had the test with nested namespace: https://github.com/denzor200/llvm-project/blob/b1bb0e351a5e2105e528dd2a509c28d30a78e025/clang-tools-extra/test/clang-tidy/checkers/misc/shadowed-namespace-function.cpp#L76
Regarding inline namespace, thanks for the nice point, I will add the test as soon as possible

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