[Serialization] Rename "alias name type" to "builtin alias type".

We still use the old layout for NameAliasType for builtin types, so
rename the Layout struct and corresponding code to describe its new
(more restricted) purpose.

include/swift/Serialization/ModuleFormat.h

@@ -641,8 +641,8 @@ namespace decls_block {
 #include "swift/Serialization/DeclTypeRecordNodes.def"
   };
 
-  using NameAliasTypeLayout = BCRecordLayout<
-    NAME_ALIAS_TYPE,
+  using BuiltinAliasTypeLayout = BCRecordLayout<
+    BUILTIN_ALIAS_TYPE,
     DeclIDField, // typealias decl
     TypeIDField  // canonical type (a fallback)
   >;