[analyzer] Use a temporary builder in CheckerContext.

[analyzer] Use a temporary builder in CheckerContext.

First step toward removing the global Stmt builder. Added several transitional methods (like takeNodes/addNodes).
+ Stop early if the set of exploded nodes for the next iteration is empty.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142827 91177308-0d34-0410-b5e6-96231b3b80d8