[RISCV] Properly diagnose mixing RVV scalable vectors with GNU vectors.

This case was being picked up by SVE code and printing an SVE
specific message.

This patch distinquishes RVV from SVE and provides a correct error
message for RVV.

The use of the generic isSizelessBuiltinType was also picking up
WebAssembly reference types which was probably an accident so I've
removed that.

I've named the test similar to SVE's test that contains this check.
Their test also tests the arm_sve_vector_bits attribute. I plan to
add something similar for RISC-V soon so I've adopted this naming.

Reviewed By: c-rhodes

Differential Revision: https://reviews.llvm.org/D144613