[Bitstream] Define VBRZ encoding, and use VBRZ6 for unabbreviated values

[sammccall]

Summary

When we don't know what we're encoding, chances are it's zero often
enough that providing a 1-bit encoding of zero is a win.

This seems to be roughly size-neutral on LLVM bitcode, and reduces Clang
PCH files by ~9%.

[MaskRay]

llvm/lib/Bitstream/Reader/BitstreamReader.cpp

110

Why is this unconditionally true?