[analyzer] Make sure base-region and its sub-regions are either all alive or all dead.

SymbolReaper now realizes that our liveness analysis isn't sharp enough
to discriminate between liveness of, say, variables and their fields.
Surprisingly, this didn't quite work before: having a variable live only
through Environment (eg., calling a C++ method on a local variable
as the last action ever performed on that variable) would not keep the
region value symbol of a field of that variable alive.

It would have been broken in the opposite direction as well, but both
Environment and RegionStore use the scanReachableSymbols mechanism for finding
live symbols regions within their values, and due to that they accidentally
end up marking the whole chain of super-regions as live when at least one
sub-region is known to be live.

It is now a direct responsibility of SymbolReaper to maintain this invariant,
and a unit test was added in order to make sure it stays that way.

Differential Revision: https://reviews.llvm.org/D56632

rdar://problem/46914108

llvm-svn: 351499