Add -Wnullability-completeness-on-arrays.

This is an addition to (and sub-warning of) -Wnullability-completeness
that warns when an array parameter is missing nullability. When the
specific warning is switched off, the compiler falls back to only
warning on pointer types written as pointer types.

Note that use of nullability /within/ an array triggers the
completeness checks regardless of whether or not the array-specific
warning is enabled; the intent there is simply to determine whether a
particular header is trying to be nullability-aware at all.

Part of rdar://problem/25846421.

llvm-svn: 286520