Auto merge of #53085 - ljedrz:cleanup_syntax_structures, r=ljedrz

Auto merge of #53085 - ljedrz:cleanup_syntax_structures, r=ljedrz

Move SmallVector and ThinVec out of libsyntax

- move `libsyntax::util::SmallVector` tests to `librustc_data_structures::small_vec`
- remove `libsyntax::util::SmallVector`
- move `libsyntax::util::thin_vec` to `librustc_data_structures::thin_vec`

Other than moving these data structures where they belong it allows modules using `SmallVector<T>` (`SmallVec<[T; 1]>`) to specify their own length (e.g. 8 or 32) independently from `libsyntax`.