[clang] Add SuppressKeywordAttrs printing policy (PR #175398)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

I don't think it make sense to treat `alignas` differently than `[[deprecated]]` in most cases - both are attributes with different spelling. A policy that hide all attributes would be easier to motivate - including for your use case.

https://github.com/llvm/llvm-project/pull/175398