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

[Luke Lau via llvm-commits]

lukel97 wrote:

Closing for now as deduplication the clang/flang frontend logic most likely requires more thought and discussion

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