[clang][RISCV] Set vscale_range attribute based on presence of "v" extension

This follows the path that AArch64 SVE has taken. Doing this via a function attribute set in the frontend is basically a workaround for the fact that several analyzes which need the information (i.e. known bits, lvi, scev) can't easily use TTI without significant amounts of plumbing changes.

This patch hard codes "v" numbers, and directly follows the SVE precedent as a result. In a follow up, I hope to drive this from RISCVISAInfo.h/cpp instead, but the MinVLen number being returned from that interface seemed to always be 0 (which is wrong), and I haven't figured out what's going wrong there.

Differential Revision: https://reviews.llvm.org/D135894