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

[Damyan Pepper via llvm-commits]

damyanp 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.

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