[clang] [llvm] [Clang][RISCV] Move getVScaleRange logic into libLLVMFrontendDriver. NFC (PR #77327)

[Alex Bradbury via cfe-commits]

asb wrote:

libLLVMFrontendDriver intuitively seems like a sensible home, though I think we'd be the first to use it in this way. I'm wondering if you considered moving the helper function into RISCVISAInfo? I'm not sure it's _better_ but it doesn't seem like a terrible place for this logic.

https://github.com/llvm/llvm-project/pull/77327