Rollup merge of #54680 - RalfJung:compile-pass, r=pnkfelix

make run-pass tests with empty main just compile-pass tests

Many run-pass tests have an empty main, so there is not actually any point in running them. This makes them `compile-pass` tests instead, saving some time (generating the binary and then running it).

For now I did this only for `run-pass/issues`; if there is interest I can also do it for the other directories. I used `^\s*fn\s+main\(\s*\)\s*\{\s*\}` as regexp to identify these files.