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

[S. Bharadwaj Yadavalli via llvm-commits]

bharadwajy wrote:

> > What are the concerns?
> 
> Have a think about how many strings will be built, and how many memory allocations there'll be, every time `sort` or `lower_bound` needs to compare two elements of the vector.

Changed to compare functions by their names instead of constructing a pseudo-signature. Non-empty Function names are unique in LLVM IR.

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