Auto merge of #89881 - Mark-Simulacrum:fieldless-fast, r=davidtwco

Avoid generating empty closures for fieldless enum variants

For many enums, this avoids generating lots of tiny stubs that need to be codegen'd and then inlined and removed by LLVM. perf shows this to be a fairly small, but significant, win on rustc bootstrap time -- with minimal impact on runtime performance (which is at times even positive).