[clang] [NFC][clang] Quote literal builtin attribute markers in Builtins.def docs (PR #160080)

[Radovan Božić via cfe-commits]

bozicrHT wrote:

> > I noticed the comment:
> > ```
> > // This is only documentation for the database layout. This will be removed once all builtin databases are converted to tablegen files  
> > ```
> > 
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> > Since it seems that not all builtins are converted yet, would it be clearer to use a less common letter (e.g. `'O'` or `'L'`) for `nonnull`, so `N` doesn’t confuse readers and avoids this change?
> 
> If we're going for a 'lower touch' thing here, I think I'd rather the current uses of `N` to not mean `nonnull` (that is, to mean "some number") could be some other markup that makes it clear. But I'm not sure what we could use reasonably.

Maybe we could take a letter from nonnull, though options are limited here :D. My proposal would be either `'O'` or `'L'`. Curious to hear what others think.

https://github.com/llvm/llvm-project/pull/160080