[llvm] [DirectX] Infrastructure to collect shader flags for each function (PR #112967)

[S. Bharadwaj Yadavalli via llvm-commits]

bharadwajy wrote:

> I stopped reading this after seeing the string construction in the comparison function. I'm either missing something big about this change, which means I'd need to review again once I've been corrected, or this PR is going to be somewhat different anyway, so I'll wait until that's resolved before looking at it again.

Lexicographic sort of functions by their signatures seemed the appropriate. I looked at using [`FunctionComparator::compareSignature()`](https://github.com/llvm/llvm-project/blob/7a710110fcb2ad5d903ec41ba6a63193cf03edc0/llvm/lib/Transforms/Utils/FunctionComparator.cpp#L946) but did not see its usage in the sources other than in tests, so wasn't sure if its usage is encouraged or not.

What are the concerns?

https://github.com/llvm/llvm-project/pull/112967