[clang] Raise an error on namespace aliases with qualified names. (PR #86122)

[via cfe-commits]

Sirraide wrote:

> ```
> test.cpp:4:20: error: namespace alias must be a single identifier
>     4 |     namespace alias::extra::qualifiers = ::myns;
>       |                    ^~~~~~~~~~~~~~~~~~~
> 1 error generated.
> ```

Nice, that is pretty much exactly what I had in mind.

https://github.com/llvm/llvm-project/pull/86122