[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>
[analyzer] Handle C string default values for const char * arguments.

Previously, SValBuilder knew how to evaluate StringLiterals, but couldn't
handle an array-to-pointer decay for constant values. Additionally,
RegionStore was being too strict about loading from an array, refusing to
return a 'char' value from a 'const char' array. Both of these have been
fixed.
[analyzer] Handle C++11 member initializer expressions.

Previously, we would simply abort the path when we saw a default member
initialization; now, we actually attempt to evaluate it. Like default
arguments, the contents of these expressions are not actually part of the
current function, so we fall back to constant evaluation.