[analyzer] Treat std::initializer_list as opaque rather than aborting.

Previously, the use of a std::initializer_list (actually, a
CXXStdInitializerListExpr) would cause the analyzer to give up on the rest
of the path. Now, it just uses an opaque symbolic value for the
initializer_list and continues on.

At some point in the future we can add proper support for initializer_list,
with access to the elements in the InitListExpr.

<rdar://problem/14340207>

llvm-svn: 186519