[clang] [CIR] Upstream pointer subtraction handling (PR #163306)

[Morris Hafner via cfe-commits]

mmha wrote:

Hey @kimsh02

it looks like you're almost there already! You're right, you need to copy the Operation definition from the incubator `CIROps.td` into your fork and add that `cir::MissingFeatures::llvmLoweringPtrDiffConsidersPointee()` assert.

https://github.com/llvm/llvm-project/pull/163306