[analyzer] Convert VisitUnaryOperator to use short lived Node builders

To convert iteratively, we take the nodes the local builder will
process from the from the global builder and add the generated nodes
after the short lived builder is done. PureStmtNodeBuilder is the
one we should eventually use everywhere. Added Stmt index and Builder
context as ExprEngine globals. To avoid passing them around.

llvm-svn: 142828