[TableGen] Ignore fake args for parsing-related arg counts.

We should complain about the following:

```
void foo() __attribute__((unavailable("a", "b")));
```

Instead, we currently just ignore "b". (...We also end up ignoring "a",
because we assume elsewhere that this attribute can only have 1 or 0
args.)

This happens because `unavailable` has a fake enum arg, and
`AttributeList::{getMinArgs,getMaxArgs}` include fake args in their
counts.

llvm-svn: 288388