[clang-format] Formatting breaks attributes with arguments

This behaviour exists up to including 21.1.6.

Formatting this piece with style llvm

``` c++
class CStruct {
public:
alignas(32) std::array, eP / Impl::eP> m_Buffs;
};
```

Results in

``` c++
class CStruct {
public:
alignas(
32) std::array, eP / Impl::eP> m_Buffs;
};
```

clang-format should try hard to not break up the attribute

Start by reproducing the formatting change with clang-format 21.1.6, using the provided C++ snippet and LLVM style. Trace the clang-format formatting path for alignas attributes with arguments; done means formatting no longer breaks the attribute across lines in this case, with a regression test covering the example.

Written by the indexing model from the issue text.