[Sema][SVE2] Move/simplify Sema testing for SVE2 ACLE builtins

Currently for SVE2 ACLE builtins, single tests are used to verify both
clang code generation (when the feature is available) and semantic
error/warning messages (when the feature is unavailable). This
patch moves the semantic testing for the target feature flag into
dedicated Sema tests.

Differential Revision: https://reviews.llvm.org/D124850