Enable @unknown default warnings by default in Swift 5 mode (#16045)

Note that I said "warnings"; we're going to be more cautious about
rollout and just make this a warning in Swift 5 mode, with /no/
diagnostics in Swift 3 and 4. Users are still free to use `@unknown
default` in these modes, and they'll get a fatal run-time error if
they don't and an unexpected case actually shows up.

rdar://problem/29324688

lib/Frontend/CompilerInvocation.cpp

@@ -312,7 +312,7 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.EnableNonFrozenEnumExhaustivityDiagnostics =
     Args.hasFlag(OPT_enable_nonfrozen_enum_exhaustivity_diagnostics,
                  OPT_disable_nonfrozen_enum_exhaustivity_diagnostics,
-                 Opts.EnableNonFrozenEnumExhaustivityDiagnostics);
+                 Opts.isSwiftVersionAtLeast(5));
 
   if (Arg *A = Args.getLastArg(OPT_Rpass_EQ))
     Opts.OptimizationRemarkPassedPattern =