The (negative) offset from a SymbolTableListTraits-using ilist to its container

The (negative) offset from a SymbolTableListTraits-using ilist to its container
object is always constant.  As such, evaluate it at compile time instead of storing
it as an ivar in SymbolTableListTraits.  This shrinks every SymbolTableListTraits
ilist by a word, shrinking BasicBlock from 44->40 bytes, Function from 96->88 bytes,
and Module from 60->52 bytes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36189 91177308-0d34-0410-b5e6-96231b3b80d8