[SE-0466] Under main actor default isolation, explicit nonisolated is not special

Given an explicitly-nonisolated type such as

    nonisolated struct S { }

all extensions of S were also being treated as nonisolated. This meant
that being implicitly nonisolated (i.e., when you're using nonisolated
default isolation) was different from explicitly-writing nonisolated,
which is unfortunate and confusing. Align the rules, such that an
extension of S will get default isolation:

    extension S {
      func f() { } // @MainActor if we're in main actor default isolation
    }

lib/Frontend/CompilerInvocation.cpp

@@ -1878,8 +1878,10 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.DefaultIsolationBehavior = DefaultIsolation::Nonisolated;
   }
 
-  if (Opts.DefaultIsolationBehavior == DefaultIsolation::MainActor)
+  if (Opts.DefaultIsolationBehavior == DefaultIsolation::MainActor) {
     Opts.enableFeature(Feature::InferIsolatedConformances);
+    Opts.enableFeature(Feature::NoExplicitNonIsolated);
+  }
 
 #if !defined(NDEBUG) && SWIFT_ENABLE_EXPERIMENTAL_PARSER_VALIDATION
   /// Enable round trip parsing via the new swift parser unless it is disabled