[analyzer] Just silence all warnings coming out of std::basic_string.

This means always walking the whole call stack for the end path node, but
we'll assume that's always fairly tractable.

<rdar://problem/15952973>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@200980 91177308-0d34-0410-b5e6-96231b3b80d8