[analyzer] Node Builder refactoring: Introduce a simple Node Builder...

[analyzer] Node Builder refactoring: Introduce a simple Node Builder responsible for generating the node frontier.

Currently we have a bunch of different node builders which provide some common
functionality but are difficult to refactor. Each builder generates nodes of
different kinds and calculates the frontier nodes, which should be propagated
to the next step (after the builder dies).

Introduce a new NodeBuilder which provides very basic node generation facilities
but takes care of the second problem. The idea is that all the other builders
will eventually use it. Use this builder in CheckerContext instead of
StmtNodeBuilder (the way the frontier is propagated to the StmtBuilder
is a hack and will be removed later on).

llvm-svn: 142443