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

[Erich Keane via cfe-commits]

erichkeane wrote:

That seems reasonable, though I'd like to hear if @Sirraide has further comments on this (or a better markup for it).  But yes, that was what I was suggesting.

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