[SelectionDAG]: Ignore "returned" in the presence of an implicit sret.

When a function return value can't be directly lowered, such as
returning an i128 on WebAssembly, as indicated by the CanLowerReturn
target hook, SelectionDAGBuilder can translate it to return the
value through a hidden sret-like argument.

If such a function has an argument with the "returned" attribute,
the attribute can't be automatically lowered, because the function
no longer has a normal return value. For now, just discard the
"returned" attribute.

This fixes PR36128.

llvm-svn: 323715