Rename check handlers to HandleParseNode overloads. (#4121)

This is for consistency with #4120. Similar to that, we can use
overloads on the typed NodeId rather than individually named handlers.
There isn't the same caller benefit here though, since the calls from
check.cpp are already boilerplate.

toolchain/check/handle_expr_statement.cpp

@@ -22,7 +22,7 @@ static auto HandleDiscardedExpr(Context& context, SemIR::InstId expr_id)
   // TODO: This will eventually need to do some "do not discard" analysis.
 }
 
-auto HandleExprStatement(Context& context, Parse::ExprStatementId /*node_id*/)
+auto HandleParseNode(Context& context, Parse::ExprStatementId /*node_id*/)
     -> bool {
   HandleDiscardedExpr(context, context.node_stack().PopExpr());
   return true;