[clang] [Sema] do not destruct fields of unions (PR #122330)

[Jameson Nash via cfe-commits]

vtjnash wrote:

This roughly corresponds to 921bd20dddf5080cdb36f39c0162eb63b2d5325e in Sema, I think (@zygoloid). Maybe @tkremenek or @yronglin has done something in this part of the code recently too as reviewers and to merge this?

https://github.com/llvm/llvm-project/pull/122330