[ADT] Teach alignment helpers to work correctly for abstract classes.

[ADT] Teach alignment helpers to work correctly for abstract classes.
This is necessary to use them as part of pointer traits and is generally
useful. I've added unit test coverage to isolate and ensure this works
correctly.

I'll watch the build bots to try to see if any compilers can't tolerate
this bit of magic (and much credit goes to Richard Smith for coming up
with this magical production!) but give a shout if you see issues.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@256553 91177308-0d34-0410-b5e6-96231b3b80d8