[clang] [clang] "modular_format" attribute for functions using format strings (PR #147431)

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

The only real 'gotcha' I can see is that thanks to the sorting, we allow 'different' attributes, as long as the 'aspects' are all the same (that is, `float, double` and `double, float`) thanks to the sort.

I question the value of that sort, and probably would just want to disallow duplicate attributes unless identical, but if you have a good reason for it, I'm Ok with it.

https://github.com/llvm/llvm-project/pull/147431