DR330: look through array types when forming the cv-decomposition of a type.

DR330: look through array types when forming the cv-decomposition of a type.

This allows more qualification conversions, eg. conversion from
   'int *(*)[]' -> 'const int *const (*)[]'
is now permitted, along with all the consequences of that: more types
are similar, more cases are permitted by const_cast, and conversely,
fewer "casting away constness" cases are permitted by reinterpret_cast.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@336745 91177308-0d34-0410-b5e6-96231b3b80d8