[TailCallElim] Remove the readonly attribute of byval.

When eliminating a tail call, we modify the values of the arguments.
Therefore, if the byval parameter has a readonly attribute, we have to remove it. It is safe because,
from the perspective of a caller, the byval parameter is always treated as "readonly," even if the readonly attribute is removed.

Fixes #64289.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D156793