DR330: when determining whether a cast casts away constness, consider

qualifiers from all levels matching a multidimensional array.

For example, this allows casting from
  pointer to       array of            array of   const volatile int
to
  pointer to const pointer to volatile pointer to                int
because the multidimensional array part of the source type corresponds
to a part of the destination type that contains both 'const' and
'volatile'.

Differential Revision: https://reviews.llvm.org/D49457

llvm-svn: 337422