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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > But I'm not sure what we could use reasonably.
> 
> We could replace it w/ e.g. `*` and add a note along the lines of ‘where `*` is any integer/index/whatever it’s actually supposed to be’

Asterisk seems pretty overloaded here... maybe the same except with `{num}` or something like that? 

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