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_function.cpp

@@ -71,8 +71,7 @@ static auto BuildFunctionDecl(Context& context, bool is_definition)
               context.sem_ir().StringifyType(return_type_id));
         })) {
       return_type_id = SemIR::TypeId::Error;
-    } else if (!SemIR::GetInitializingRepresentation(context.sem_ir(),
-                                                     return_type_id)
+    } else if (!SemIR::GetInitRepr(context.sem_ir(), return_type_id)
                     .has_return_slot()) {
       // The function only has a return slot if it uses in-place initialization.
     } else {