[CIR] Upstream initial support for CIR flattening (#130648)
The ClangIR CFG has to be flat before it can be lowered to LLVM IR. That
is, there can be no nested regions and all blocks in a region must
belong to the parent region. Currently only cir.scope operations violate
these rules, so the initial implementation of the cir-flatten-cfg pass
only has to transform scope operations.