[clang][AArch64][SVE] Change SVE_VECTOR_OPERATORS macro for VLA vectors

The __ARM_FEATURE_SVE_VECTOR_OPERATORS macro should be changed to
indicate that this feature is now supported on VLA vectors as well as
VLS vectors. There is a complementary PR to the ACLE spec here
https://github.com/ARM-software/acle/pull/213

Reviewed By: peterwaller-arm

Differential Revision: https://reviews.llvm.org/D131573

(cherry picked from commit 13a784f3)