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

[Kim Gräsman via cfe-commits]

kimgr 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.

Good idea, thanks. I posted a draft commit here: https://github.com/kimgr/llvm-project/commit/declprinter-suppress-all

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