base classes
`ExprLiteral::v` needs to be initialized in ExprLiteral's constructor,
but at the time that it gets initialized subclass fields don't yet exist
so it can't reference them. Previously, `v` was default constructed and
then later assigned a proper value, but this creates a problem when
attempting to remove all default constructions of `Value` from the
codebase. This commit moves the backing fields of each of the subclasses
into new base classes, which exist just to make sure they get
initialized before the `ExprLiteral` base class.
Work towards #744
Change-Id: Ic6d24cab474460b113f2fbcc8d92ad266a6a6964
skye
cf5e5f599e