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.                   

llvm-svn: 124785