[clang] [CIR] Add lowering for long-double increment/decrement (PR #190812)

[Andy Kaylor via cfe-commits]

https://github.com/andykaylor approved this pull request.

This is good for the immediate problem with long doubles. We should follow up with something that generalizes the FP type handling here. The biggest thing it will need is tests.

https://github.com/llvm/llvm-project/pull/190812