[clang-tools-extra] [clang-tidy] Fix `cppcoreguidelines-use-enum-class` anonymous enum bug (PR #215352)

[Baranov Victor via cfe-commits]

vbvictor wrote:

> I did in [89e11ad](https://github.com/llvm/llvm-project/commit/89e11adb8cb7a463ae2f5c460e05759c63161086), or perhaps the wording should be different?

We should update here
https://clang.llvm.org/extra/clang-tidy/checks/cppcoreguidelines/use-enum-class.html#cppcoreguidelines-use-enum-class

"Finds unscoped (non-class) enum declarations and suggests using enum class instead."
Add something like "Unnamed enum are ignored and will be handled by check implementing [Enum.6](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#enum6-avoid-unnamed-enumerations)"

And later we update docs with real link to check implementing Enum.6 check

https://github.com/llvm/llvm-project/pull/215352