Add test coverage for array to pointer decay in non-type template parameters.

Functionality committed in r172585 but tested the function case without the
array case.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@176215 91177308-0d34-0410-b5e6-96231b3b80d8
PR15360: nullptr as a non-type template argument to a function type non-type template parameter

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@176216 91177308-0d34-0410-b5e6-96231b3b80d8