[clang][RISCV] Set vscale_range attribute based on VLEN

Follow up on D135894, restructure code to work in terms of minimum and maximum VLEN coming from RISCVISAInfo.cpp. In the original review, I'd mentioned that MinVLEN was sometimes zero. This turns out to be a case of human error, combined with really bad (lack of) error reporting.

This patch adds appropriate tests for various vector extension combinations to show the mechanism works, but doesn't try to provide exhaustive coverage of the extension interactions. Presumably, that is already covered in existing tests elsewhere.

Differential Revision: https://reviews.llvm.org/D136106