[clang][aarch64] Drop experimental from __ARM_FEATURE_SVE_BITS macro
The ARM_FEATURE_SVE_BITS feature macro is specified in the Arm C
Language Extensions (ACLE) for SVE  (version 00bet5). From the spec,
When N is nonzero, indicates that the implementation is generating code for an N-bit SVE target and that the arm_sve_vector_bits(N) attribute is available.
This was defined in D83550 as __ARM_FEATURE_SVE_BITS_EXPERIMENTAL and
enabled under the -msve-vector-bits flag to simplify initial tests.
This patch drops _EXPERIMENTAL now there is support for the feature.
Reviewed By: david-arm
Differential Revision: https://reviews.llvm.org/D86720