Abbreviate "representation" -> "repr" (#3464)

Specifically using these abbreviations:
* InitRepr: "initializing representation"
* ObjectRepr: "object representation"
* ValueRepr: "value representation"

As discussed in [#toolchain
discord](https://discord.com/channels/655572317891461132/655578254970716160/1182086098470572143)
and now documented in the [list of
abbreviations](https://docs.google.com/document/d/1RRYMm42osyqhI2LyjrjockYCutQ5dOf8Abu50kTrkX0/edit?resourcekey=0-kHyqOESbOHmzZphUbtLrTw#heading=h.pph7i5m5un7q).

toolchain/check/handle_class.cpp

@@ -280,7 +280,7 @@ auto HandleClassDefinition(Context& context, Parse::NodeId parse_node) -> bool {
 
   // The class type is now fully defined.
   auto& class_info = context.classes().Get(class_id);
-  class_info.object_representation_id =
+  class_info.object_repr_id =
       context.CanonicalizeStructType(parse_node, fields_id);
   return true;
 }