Diagnostic: Provide end-user message for aliasing errors

This will spill out information about LLVM-internals. However, in cases
where the name of the Value matches the name of the array in the source,
we provide more useful information. In cases where we spill internals,
the information still might help the user to pin down the correct
arrays.
The problem we face here is: The error is pinned to the debug location
of one of the offending values out of the alias set instead of all of them.

The more information we give the user about the set of aliasing
pointers the better.

llvm-svn: 215830