[llvm] [MC][ARM] Don't set funcs to Thumb as a side effect of .hidden (PR #181156)

[Peter Smith via llvm-commits]

https://github.com/smithp35 approved this pull request.

Implementation looks good to me. GNU arm-none-eabi-as matches the proposed behaviour .hidden doesn't affect Arm/Thumb state with the test case.

Looking at all the MCSA_ attributes, the only ones I can see that I would expect to change the Arm/Thumb state are `MCSA_ELF_TypeFunction` and `MCSA_ELF_TypeIndFunction`. The only other that looks close is `MCSA_IndirectSymbol` but that is MachO specific and not handled by the ELF Streamers.

There's scope for adding tests to make sure that all the other MCSA_ directives don't affect the thumb status but I don't think that is required.

Worth giving other reviewers some time to comment before merging.

https://github.com/llvm/llvm-project/pull/181156