Update error message for invalid operand token while parsing operand list.

Previously the error could mislead into thinking it was a parser bug instead of the input being erroneous. Update to make it clearer.

PiperOrigin-RevId: 212271145
diff --git a/lib/Parser/Parser.cpp b/lib/Parser/Parser.cpp
index a2a45e7..5641421 100644
--- a/lib/Parser/Parser.cpp
+++ b/lib/Parser/Parser.cpp
@@ -1796,9 +1796,7 @@
       if (parser.getToken().is(Token::l_paren) ||
           parser.getToken().is(Token::l_square))
         return emitError(startLoc, "unexpected delimiter");
-      return emitError(startLoc, "unable to parse '" +
-                                     parser.getTokenSpelling() +
-                                     "' as operand");
+      return emitError(startLoc, "invalid operand");
     case Delimiter::OptionalParen:
       if (parser.getToken().isNot(Token::l_paren))
         return false;