Refactor SymbolTableListTraits to only have a single pointer in it, instead

Refactor SymbolTableListTraits to only have a single pointer in it, instead
of two.  This shrinkifies Function by 8 bytes (104->96) and Module by 8
bytes (68->60).  On a testcase of mine, this reduces the memory used to
read a module header from 565680b to 561024, a little over 4K.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36188 91177308-0d34-0410-b5e6-96231b3b80d8