ADT: Tidy up ilist_traits static asserts, NFC

Change the ilist traits to use decltype instead of sizeof, and add
HasObsoleteCustomization so that additions to this list don't need to be
added in two places.

I suspect this will now work with MSVC, since the trait tested in
r278991 seems to work.  If for some reason it continues to fail on
Windows I'll follow up by adding back the #ifndef _MSC_VER.

llvm-svn: 279012