[clang] [CIR] Handle an empty C++ class in x86_64 callconv lowering (PR #214742)

[Adam Smith via cfe-commits]

adams381 wrote:

The padding rework came out bigger than I hoped.  I broke it down as best I could, but it is a stack of three large PRs: #215174 gives the record type a kind per member, #215175 has CIRGen fill them in, and #215176 retires the record-level `padded` bool and derives it from the marks.

Once that stack lands, this PR gets rebased on #215176 and drops `is_empty` for a query over the marks.

https://github.com/llvm/llvm-project/pull/214742