Recommit [NFC] [Serialization] Packing more bits and refactor AbbrevToUse

This patch tries to pack more bits into a value to reduce the size of
.pcm files. Also, after we introduced BitsPackers, it may slightly
better to adjust the way we use Abbrev.

After this patch, the size of the BMI for std module reduce from 28.94MB
to 28.1 MB.

This was reverted due to it broke the build of lldb. The reason that we
skip the serialization of a source location incorrectly. And this patch
now fixes that.