[clang] [CIR] Upstream initial support for unary op (PR #131369)

[Bruno Cardoso Lopes via cfe-commits]

bcardosolopes wrote:

> The git history says the FIXME about "Pure" was added when @cmarcelo introduced `UnaryOp` in the incubator, but there is no PR associated with the commit so I don't know what discussions may have taken place.

We floated the idea of implementing operators as part of `UnaryOp`. This was a while ago and it probably doesn't make sense and is outdated!

https://github.com/llvm/llvm-project/pull/131369