[Concurrency] NonisolatedNonsendingByDefault: Don't attempt to migrate $ prefixed declarations

These are special declarations that are synthesized by the compiler
or a macro and warnings about them are non-actionable.

Author: xedin

include/swift/AST/Identifier.h

@@ -362,7 +362,7 @@ class DeclBaseName {
   }
 
   bool hasDollarPrefix() const {
-    return getIdentifier().hasDollarPrefix();
+    return !isSpecial() && getIdentifier().hasDollarPrefix();
   }
 
   /// A representation of the name to be displayed to users. May be ambiguous

lib/Sema/NonisolatedNonsendingByDefaultMigration.cpp

@@ -105,6 +105,12 @@ void NonisolatedNonsendingByDefaultMigrationTarget::diagnose() const {
       return;
     }
 
+    // A special declaration that was either synthesized by the compiler
+    // or a macro expansion.
+    if (decl->getBaseName().hasDollarPrefix()) {
+      return;
+    }
+
     // If the attribute cannot appear on this kind of declaration, we can't
     // diagnose it.
     if (!DeclAttribute::canAttributeAppearOnDecl(DeclAttrKind::Concurrent,