[clang] [clang] Add test for CWG1606 (PR #87274)

[Shafik Yaghmour via cfe-commits]

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

Thank you for the test! LGTM.

Do we also have a test that `sizeof([=]{ return i + j;})` should fail as well? Tangentially related to this DR but if we don't we should cover that in our tests someplace.

https://github.com/llvm/llvm-project/pull/87274