More capturing of 'this': implicit member expressions. Getting that

More capturing of 'this':  implicit member expressions.  Getting that
right for anonymous struct/union members led to me discovering some
seemingly broken code in that area of Sema, which I fixed, partly by
changing the representation of member pointer constants so that
IndirectFieldDecls aren't expanded.  This led to assorted cleanups with
member pointers in CodeGen, and while I was doing that I saw some random
other things to clean up.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@124785 91177308-0d34-0410-b5e6-96231b3b80d8