[analyzer] NoStoreFuncVisitor: Suppress reports with no-store in system headers.

The idea behind this heuristic is that normally the visitor is there to
inform the user that a certain function may fail to initialize a certain
out-parameter. For system header functions this is usually dictated by the
contract, and it's unlikely that the header function has accidentally
forgot to put the value into the out-parameter; it's more likely
that the user has intentionally skipped the error check.

Warnings on skipped error checks are more like security warnings;
they aren't necessarily useful for all users, and they should instead
be introduced on a per-API basis.

Differential Revision: https://reviews.llvm.org/D60107

llvm-svn: 357810