Lay the groundwork for my upcoming ilist sentinel shrinking patch

Lay the groundwork for my upcoming ilist sentinel shrinking patch
by defining a LLVM_COMPACT_SENTINELS symbol to 0 or 1 in config.h.

I'm asking for 3 favors:
- may an autofoo expert look at this and suggest improvements?
- may a cmake expert suggest analogous functionality for config.h?
- may somebody with the right autofoo mix regenerate configure? (mine is too new)

Thanks!
--This line, and those below, will be ignored--

M    configure.ac

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78628 91177308-0d34-0410-b5e6-96231b3b80d8