[clang] [WebAssembly] Enable musttail only when tail-call is enabled (PR #163618)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! It's not quite ready, but I think it's heading in the right direction.

As we get closer to finalizing it, this should also come with a release note in `clang/docs/ReleaseNotes.rst` so users know about the fix.

https://github.com/llvm/llvm-project/pull/163618