[clang] [llvm] [ARM] musttail fixes (PR #102896)

[via cfe-commits]

kiran-isaac wrote:

Really made a mess of it here. For anyone trying to understand what on earth happened:
- I pushed a commit to main rather than pushing to the branch on my fork. Lesson learned to always use git in the command line rather than pressing the "sync" button on vscode
- I reverted the commit on main, and then merged from main, reverting the changes
- I then force pushed to drop the revert commit rather than reverting the revert commit
- I realised that I had lost my original changes in the chaos, so I did one more force push to cherry pick the two commits I actually wanted rather.

Apologies for any confusion. 

https://github.com/llvm/llvm-project/pull/102896