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

[Andy Kaylor via cfe-commits]

andykaylor 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.

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