llvm-cov: Distinguish expansion/instantiation from SourceCoverageView

SourceCoverageView currently has "Kind" and a list of child views, all
of which must have either an expansion or an instantiation Kind. In
addition to being an error-prone design, this makes it awkward to
differentiate between the two child types and adds a number of
optionally used members to the type.

Split the subview types into their own separate objects, and maintain
lists of each rather than one combined "Children" list.