uber / NullAway / #874
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Add support for Preconditions.checkArgument through custom CFG translation (#608)

See #47 for discussion.

This PR adds support for `Preconditions.checkArgument(...)` and `Preconditions.checkState(...)`
throw a new `PreconditionsHandler` handler. 

In order to implement said handler, we need a custom CFG translation pipeline in NullAway 
(`NullAwayCFGBuilder`) which subclasses the Checker Framework's `CFGBuilder` 
(which we were using directly before this change). This pipeline allows us to add handler
callbacks during the AST to CFG translation process. At the moment, we add a single
such callback, right after visiting a `MethodInvocationTree`. We also add a more or less
generic method to insert a conditional jump and a throw based on a given boolean expression
node.

Abstracting some details about our AST and CFG structures, we translate:

```
Preconditions.checkArgument($someBoolExpr[, $someString]);
``` 

Into something resembling:

```
Preconditions.checkArgument($someBoolExpr[, $someString]);
if ($someBoolExpr) {
   throw new IllegalArgumentException();
}
``` 

Note that this causes `$someBoolExpr` to be evaluated twice. This is necessary based on how
NullAway evaluates branch conditionals, since we currently do not track boolean values through
our dataflow (e.g. `boolean b = (o == null); if (b) { throw [...] }; o.toString();` produces a dereference
according to NullAway, independent on whether that code was added by rewriting or directly on the
source. Obviously, this doesn't change the code being compiled or bytecode being produced,
the rewrite is only ever used by/visible to NullAway itself.

Finally, our implementation of `NullAwayCFGBuilder` and particularly `NullAwayCFGTranslationPhaseOne`
in this PR, depend on a Checker Framework APIs that are currently private. We are simultaneously
submitting a PR to change the visibility of these APIs (see [CheckerFramework#5156](https://github.com/typetools/checke... (continued)

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