[Clang] `gnu::aligned` attribute inconsistently removed during type un-qualification

https://godbolt.org/z/a1zjYMrY5
```c
[[gnu::aligned]] typedef int T1;
[[gnu::aligned]] typedef int const T2;
[[gnu::aligned]] typedef int volatile T3;

__typeof_unqual(T1) i1; // 16-bytes aligned
__typeof_unqual(T2) i2; // 4-bytes aligned
__typeof_unqual(T3) i3; // 4-bytes aligned
```
IMO this is rather inconsistent, however GCC also behaves like this.

Start with the linked Godbolt reproducer and compare Clang's handling of __typeof_unqual for T1, T2, and T3, including the resulting alignment. The issue does not name a source file or test, and it leaves the intended consistent behavior unresolved; done would require an agreed behavior plus coverage demonstrating it.

Written by the indexing model from the issue text.