Deal with new warnings in Clang 21 / Xcode 26.4

[snej]

Every Clang release adds new warnings, which turn into errors in our LITECORE_WARNINGS_HARDCORE build setting (and sometimes in Xcode builds too.) Some warnings I've disabled in the config, some I've fixed in source like

• Unnecessary semicolons
• Initializing an enum with 0
• Unnecessary 'virtual' on methods in a final class
• Implicit fallthrough between switch cases
• Some implicit int/float comparisons, although there were a lot more and I had to suppress the warning in the end
• A weird one in SmallVectorBase.hh that warned that the MS ABI wouldn't pack bitfields properly unless I made them the same type
• Const string arrays not declared constexpr