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

[Kim Gräsman via cfe-commits]

kimgr wrote:

@vgvassilev This will allow IWYU to do a minimum of trivial post-processing to transform printed record decls to valid forward declarations. Not sure if it's OK for upstream, but the more I look at handling it downstream, the more potential issues I find.

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