Shift Parser from 'Identifier' to 'Name' naming (#2947)

This PR renames parse nodes on a Name/NameExpression taxonomy. NameExpressions occur in a name context. The difference is that in non-expression contexts it's useful to return the identifier / string ID for adding to name lookup, whereas in expression contexts it's useful to return the resolved node ID for consistency with other expressions.

In the code, I do note SelfValueName is returned in the expression context: I'd expect this to change, as `self` in `[self: Self]` versus `self.Foo()` will probably be best handled similarly to the above. That means that, in the proposed taxonomy, both `SelfValueName` and `SelfValueNameExpression` will exist in order to assist semantics.

To contrast choices:

Original | Current | [zygoloid suggestion](https://discord.com/channels/655572317891461132/655578254970716160/1121581663399464970) | [This PR](https://discord.com/channels/655572317891461132/655578254970716160/1121814551789318215)
--- | --- | --- | ---
DeclaredName/DesignatedName | Identifier | NameComponent | Name
NameReference | NameReference | NameReference | NameExpression
SelfValueIdentifier | SelfValueIdentifier | SelfValueReference | SelfValueName
SelfTypeIdentifier | SelfTypeIdentiifer | SelfTypeReference | SelfTypeNameExpression

toolchain/semantics/semantics_handle_struct.cpp

@@ -21,7 +21,7 @@ auto SemanticsHandleStructFieldDesignator(SemanticsContext& context,
   // This leaves the designated name on top because the `.` isn't interesting.
   CARBON_CHECK(
       context.parse_tree().node_kind(context.node_stack().PeekParseNode()) ==
-      ParseNodeKind::Identifier);
+      ParseNodeKind::Name);
   return true;
 }
 
@@ -33,7 +33,7 @@ auto SemanticsHandleStructFieldType(SemanticsContext& context,
 
   auto [name_node, name_id] =
       context.node_stack().PopWithParseNode<SemanticsStringId>(
-          ParseNodeKind::Identifier);
+          ParseNodeKind::Name);
 
   context.AddNode(
       SemanticsNode::StructTypeField::Make(name_node, cast_type_id, name_id));
@@ -51,7 +51,7 @@ auto SemanticsHandleStructFieldValue(SemanticsContext& context,
   auto [value_parse_node, value_node_id] =
       context.node_stack().PopWithParseNode<SemanticsNodeId>();
   auto name_id =
-      context.node_stack().Pop<SemanticsStringId>(ParseNodeKind::Identifier);
+      context.node_stack().Pop<SemanticsStringId>(ParseNodeKind::Name);
 
   // Store the name for the type.
   auto type_block_id = context.args_type_info_stack().PeekForAdd();