Merge pull request #36313 from ahoppen/pr/custom-opt-storage

[libSyntax] Hide null AbsoluteRawSyntax behind a custom OptionalStorage implementation

include/swift/Syntax/RawSyntax.h

@@ -526,6 +526,7 @@ public:
   /// Get a child based on a particular node's "Cursor", indicating
   /// the position of the terms in the production of the Swift grammar.
   const RawSyntax *getChild(CursorIndex Index) const {
+    assert(Index < getNumChildren() && "Index out of bounds");
     return getLayout()[Index];
   }