[clang] [clang][bytecode] Make union activation more granular (PR #148835)

[Timm Baeder via cfe-commits]

tbaederr wrote:

A little better: https://llvm-compile-time-tracker.com/compare.php?from=952f225a7b575f1a431f8eed391605e7acc0d072&to=7b226fba8d1ff31356b6a6a62ee810ed8d7f98fd&stat=instructions:u

https://github.com/llvm/llvm-project/pull/148835