[llvm] [AArch64][Build Attributes] Improve functionality and fix bugs (PR #126530)

[Oliver Stannard via llvm-commits]

ostannard wrote:

Would you mind splitting this into multiple patches to make it easier to review? Based on the PR description, this looks like it could be three patches:
* Improve testing of the existing behaviour, no functional changes
* Do not allow unknown tags to be strings, and add/change tests for this
* Remove assertion when value exists, and add/change tests for this

I think these are closely related enough to stay in one PR, but splitting them into patches allows me (and other reviewers) to see which test changes correspond to which code changes.

https://github.com/llvm/llvm-project/pull/126530