[clang] [llvm] [ARM] Fix musttail calls (PR #109943)

[Eli Friedman via cfe-commits]

================
@@ -12658,10 +12658,10 @@ This instruction requires several arguments:
       the return value of the callee is returned to the caller's caller, even
       if a void return type is in use.
 
-   Both markers imply that the callee does not access allocas from the caller.
-   The ``tail`` marker additionally implies that the callee does not access
-   varargs from the caller. Calls marked ``musttail`` must obey the following
-   additional  rules:
+   Both markers imply that the callee does not access allocas or ``byval``
+   arguments from the caller. The ``tail`` marker additionally implies that the
----------------
efriedma-quic wrote:

This is fine, but please submit separately.

https://github.com/llvm/llvm-project/pull/109943