[clang] [clang] Add test for CWG1807 (PR #77637)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> This brings me to conclusion that hand-written style of codegen tests presented above is a better fit for purposes of C++ defect report codegen tests.

I share that conclusion. I think the hand-written IR is easier for people to reason because it strips out unrelated details that make the test somewhat more brittle than it needs to be for DR testing. I agree that we should use the script whenever possible, but DR testing is a bit different from typical codegen testing IMO. So I'm fine with the hand-crafted IR unless I'm missing something.

https://github.com/llvm/llvm-project/pull/77637