Fix a rejects-valid in C++11: array new of a negative size, or overflowing array

new, is well-formed with defined semantics of throwing (a type which can be
caught by a handler for) std::bad_array_new_length, unlike in C++98 where it is
somewhere nebulous between undefined behavior and ill-formed.

If the array size is an integral constant expression and satisfies one of these
criteria, we would previous the array new expression, but now in C++11 mode, we
merely issue a warning (the code is still rejected in C++98 mode, naturally).

We don't yet implement new C++11 semantics correctly (see PR11644), but we do
implement the overflow checking, and (for the default operator new) convert such
expressions to an exception, so accepting such code now does not seem especially
unsafe.

llvm-svn: 149767