[clang][Interp][NFC] Add missing comment

Looks like I lost this last time.

Author: tbaederr

clang/lib/AST/Interp/ByteCodeEmitter.cpp

@@ -22,6 +22,11 @@
 using namespace clang;
 using namespace clang::interp;
 
+/// Unevaluated builtins don't get their arguments put on the stack
+/// automatically. They instead operate on the AST of their Call
+/// Expression.
+/// Similar information is available via ASTContext::BuiltinInfo,
+/// but that is not correct for our use cases.
 static bool isUnevaluatedBuiltin(unsigned BuiltinID) {
   return BuiltinID == Builtin::BI__builtin_classify_type;
 }