[InferAttrs] Do not mark first argument of str(n)cat as writeonly.

str(n)cat appends a copy of the second argument to the end of the first
argument. To find the end of the first argument, str(n)cat has to read
from it until it finds the terminating 0. So it should not be marked as
writeonly. I think this means the argument should not be marked as
writeonly.

(This is causing a mis-compile with legacy DSE, before it got removed)

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D100601