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

[Sam Elliott via llvm-commits]

https://github.com/lenary approved this pull request.

I'm happy with how this works. 

I do think we have to wait for a little more progress on the spec documents before landing it.

We should also document all of clang's supported tune options in RISCVUsage.rst, if we can.

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