[nfc] Add comment describing when there may not be a parent source file.

Add a comment explaining that C++ function wont have an attached parent
source file. Refs swiftlang#35311.

Author: zoecarver

lib/AST/ASTScopeCreation.cpp

@@ -209,6 +209,8 @@ void ASTScope::
 }
 
 void ASTScope::expandFunctionBody(AbstractFunctionDecl *AFD) {
+  // There is no source file associated with C++ decl contexts, so there will
+  // be no parent source file if AFD is a C++ function.
   if (auto *const SF = AFD->getParentSourceFile())
     SF->getScope().expandFunctionBodyImpl(AFD);
 }