Auto merge of #39063 - ruuda:covered-switch, r=alexcrichton

Fix covered-switch-default warnings in RustWrapper

These switch statements cover all possible values, so the default case is dead code (it contains an `llvm_unreachable anyway`), triggering a `-Wcovered-switch-default` warning that pollutes the build output. Moving the unreachable after the switch resolves these warnings.

r? @rkruppe