There are 4 cases across 2 files (mallocSiteTable.cpp and objectMonitor.cpp) where empty-body warnings cause compilation failure on recent versions of Xcode. We should fix these as part of our toolchain upgrade effort.
This is a partial backport of Paul Hohensee's massive cleanup  of Xcode warnings. A complete backport of that chnageset is well beyond the scope of our toolchain upgrade work. But we do want to clean up as many warnings as we can with minimal risk.