[llvm] [RISCV] Introduce a new tune feature string syntax and its parser (PR #168160)

[Craig Topper via llvm-commits]

topperc wrote:

SFB is turning into a bit of a complex feature to express. I think it's currently 3 tune features, with a 4th in review. The current granularity is tied to implementations we know of and the names of LLVM scheduler classes. I don't know if that's the granularity we would want to expose to users. But I guess maybe that's just a documentation issue.

https://github.com/llvm/llvm-project/pull/168160