[clang] [CIR] Add support for ternary operator as lvalue (PR #163580)

[Morris Hafner via cfe-commits]

mmha wrote:

The problem with throw exprs is that they lower to `cir.throw` + `cir.unreachable` as the terminator of a split block. `TernaryOp` on the other hand expects both branches to terminate with a `YieldOp`. Adding any instructions past `unreachable` fails verification. So I changed `FlattenCFG` to accept either `yield` or `unreachable` as a terminator. If anyone can think of a better solution let me know

https://github.com/llvm/llvm-project/pull/163580