[clang] [ARM] Change NEON poly type to be unsigned (#56781) (PR #80691)

[Alfie Richards via cfe-commits]

AlfieRichardsArm wrote:

This could be an ABI break. `poly8_t` will now be `uint8_t` rather than `int8_t`, which also changes the signatures of certain functions. Certainly there were some comments in the effected code as to why this was not done initially referring to ABI.

@mmalcomson I was sent your name as someone who might be able to provide some wisdom.

https://github.com/llvm/llvm-project/pull/80691