[BuildLibCalls] Add argmemonly, writeonly, nounwind to memset_chk.

The memset_chk library function should match memset's attributes with
respect of memory effects (argmemonly, writeonly). It also does not
raise exceptions. It may not return, in case it aborts the program.

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D114793