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_codeblock.cpp

@@ -7,14 +7,14 @@
 
 namespace Carbon::Check {
 
-auto HandleCodeBlockStart(Context& context, Parse::CodeBlockStartId node_id)
+auto HandleParseNode(Context& context, Parse::CodeBlockStartId node_id)
     -> bool {
   context.node_stack().Push(node_id);
   context.scope_stack().Push();
   return true;
 }
 
-auto HandleCodeBlock(Context& context, Parse::CodeBlockId /*node_id*/) -> bool {
+auto HandleParseNode(Context& context, Parse::CodeBlockId /*node_id*/) -> bool {
   context.scope_stack().Pop();
   context.node_stack()
       .PopAndDiscardSoloNodeId<Parse::NodeKind::CodeBlockStart>();