[CIR] Upstream unary expression support

[andykaylor]

Overview

Upstream support for unary expressions and the cir.unary operation. The initial change should just add support a single operation, such as not. Other unary operators can then be added afterwards.

Suggested minimal test case

bool f(bool a) {
  return !a;
}

Existing incubator tests

clang/test/CIR/CodeGen/unary.cpp

[andykaylor]

I've started work on this. It's currently blocked by cast operation support, but I expect to post a PR soon after that lands.

[andykaylor]

Initial support was added here: #131369

Type promotion still needs to be handled.

[andykaylor]

A few follow-up pieces are up for review now.

NSW handling (thanks, @Lancern, for implementing this in the incubator!)
#133815

Type promotion (which may need to be deferred to LLVM IR lowering)
#133829

Logical not
#133966

I intend to open a separate issue for other unary C/C++ operators, such as & and * because they'll be lowered to different CIR operations.