Auto merge of #49890 - varkor:xpy-check-rustc_trans, r=alexcrichton

Auto merge of #49890 - varkor:xpy-check-rustc_trans, r=alexcrichton

Add rustc_trans to x.py check

r? @Mark-Simulacrum

I looked at `bootstrap/compile.rs` and `bootstrap/check.rs` to try to work out which steps were appropriate, but I'm sure I've overlooked some details here, so it's worth checking carefully I've got all the steps right (e.g. I wasn't sure whether we want to build LLVM if necessary with `x.py check`, though I thought it was probably better to than to not).

From a quick test, it seems to be working, though.