[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  
```
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?


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