Make the Size and DataSize members more CharUnits-friendly by wrapping them

with getter and setter methods in both bit units and CharUnits. This will help
simplify some of the unit mismatch in the parts of the code where sizes are
known to be exact multiples of the width of the char type.

Assertions in the getters help guard against accidentally converting to
CharUnits when sizes are not exact multiples of the char width.

llvm-svn: 126354