[AArch64][SME] Don't infer -neon from +streaming-sve.

In Streaming SVE mode full NEON is not available, even though this is
implied from armv8-a. LLVM previously inferred that NEON needed to be
disabled when setting +streaming-sve, but there is no need to infer
this from +streaming-sve, because we can explicitly disable NEON using
LLVM's attribute mechanism. This is specifically relevant because
+streaming-sve is not a user-facing feature, but rather an LLVM internal
feature.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D120809