[clang] Revert "[DebugInfo] Ignore undefined constexpr constructors in constructor homing." (PR #221566)

[Avi Kivity via cfe-commits]

avikivity wrote:

This common initial subsequence rule, https://timsong-cpp.github.io/cppwp/n3337/class.mem#19, which explicitly says it's allowed to construct via one arm of the union and read via the other.

https://github.com/llvm/llvm-project/pull/221566