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

[Daniel M. Katz via cfe-commits]

katzdm wrote:

@Sirraide Thanks for the quick review! I've applied your feedback. Here is an example error message with the patch applied:
```
test.cpp:4:20: error: namespace alias must be a single identifier
    4 |     namespace alias::extra::qualifiers = ::myns;
      |                    ^~~~~~~~~~~~~~~~~~~
1 error generated.
```

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