Stat system dependencies when using -verify-pch

We don't stat the system headers to check for stalenes during regular
PCH loading for performance reasons.  When explicitly saying
-verify-pch, we want to check all the dependencies - user or system.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@200979 91177308-0d34-0410-b5e6-96231b3b80d8
[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: http://llvm.org/svn/llvm-project/cfe/trunk@200980 91177308-0d34-0410-b5e6-96231b3b80d8