Change the builtin enum approach to enforce addition. (#2584)

It feels like every time I add a builtin, I forget to add it to the builtin IR. This approach enforces the addition of builtin nodes, and also takes advantage of the normalized structure to set types appropriately (I think the new approach is simpler on this aspect).

toolchain/semantics/semantics_builtin_kind.cpp

@@ -7,7 +7,8 @@
 namespace Carbon {
 
 CARBON_DEFINE_ENUM_CLASS_NAMES(SemanticsBuiltinKind) = {
-#define CARBON_SEMANTICS_BUILTIN_KIND(Name) CARBON_ENUM_CLASS_NAME_STRING(Name)
+#define CARBON_SEMANTICS_BUILTIN_KIND_NAME(Name) \
+  CARBON_ENUM_CLASS_NAME_STRING(Name)
 #include "toolchain/semantics/semantics_builtin_kind.def"
 };