add a denser encoding for null terminated strings, add a 6-bit abbrev as

add a denser encoding for null terminated strings, add a 6-bit abbrev as
well.  This shrinks kc++ from 2724088 to 2717360 bytes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@36821 91177308-0d34-0410-b5e6-96231b3b80d8