[clang] [FMV][AArch64][clang] Emit fmv-features metadata in LLVM IR. (PR #118544)

[Alexandros Lamprineas via cfe-commits]

labrinea wrote:

> > Suppose you have target_version("i8mm+dotprod") and target_version("fcma").  The first version clearly has higher priority
> 
> According to the [current rules](https://arm-software.github.io/acle/main/acle.html#selection) yes, but IMO that rule is broken and doesn't match user expectation. A motivating example might be `target_version("simd+dotprod")` and `target_version("sve")`: when sve is available we _should_ pick that version, but the current rule prioritizes the _number_ of attribute-specified features over their overall weight.
> 
> I think if we fix that rule, and instead sort priorities lexicographically after adding implicit features implied by command-line options, then this change becomes unnecessary. Mind starting a thread with the gcc folks to sort this out?

@andrewcarlotti is planning a PR in ACLE to fix the priorities. I will tag you there when this happens.

https://github.com/llvm/llvm-project/pull/118544