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

[Jon Roelofs via cfe-commits]

jroelofs 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?

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