[RISCV] Add support for the vscale_range attribute.

This is based on @frasercrmck's D107290. At least some of the clang
portion of D107290 has already been committed.

This uses vscale_range for min/max vector width unless the command
line overrides are used.

As a follow up, I plan to add a max or exact VLEN option to clang
to control the vscale_range. This will eliminate many of the reasons
for users to use the overrides through the -mllvm interface.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D139873