analyzer: Introduce a new class, ValueManager, that serves as an aggregate

"manager of symbolic values", wrapping BasicValueFactory, SymbolManager, and
MemRegionManager. While these individual managers nicely separate functionality
in the analyzer, constructing symbolic values can sometimes be cumbersome
because it requires using multiple managers at once. The goal of this class is
to create some factory methods to create SVals that require the use of these
different managers, thus (hopefully) simplifying the analyzer API for clients.

llvm-svn: 68709