[analyzer] Assume 'new' never returns NULL if it could throw an exception.

This is actually required by the C++ standard in
[basic.stc.dynamic.allocation]p3:

  If an allocation function declared with a non-throwing
  exception-specification fails to allocate storage, it shall return a
  null pointer. Any other allocation function that fails to allocate
  storage shall indicate failure only by throwing an exception of a type
  that would match a handler of type std::bad_alloc.

We don't bother checking for the specific exception type, but just go off
the operator new prototype. This should help with a certain class of lazy
initalization false positives.

<rdar://problem/12115221>

llvm-svn: 166363