StreamToString fails if supertype of used type is not visible

### What version of Error Prone are you using?
2.2.0

### Does this issue reproduce with the latest release?
Yes

### What did you do?

Error-prone requires entire parent

If possible, provide a recipe for reproducing the error.
A complete small program is good.

1) Have class A extends B
2) Compile class B
3) Compile class A (against B)

4) Construct Class that has
`.toString()`

5) Attempt to compile it with EP.

### What did you expect to see?

Success (as Javac will produce)

### What did you see instead?

tooling/experimental/thrift-genrules/dependency/JavaClient.java:7: error: An unhandled exception was thrown by the Error Prone static analysis plugin.
System.out.println(FooBar.BAR);
^
Please report this at https://github.com/google/error-prone/issues/new and include the following:

error-prone version: 2.2.0
Stack Trace:
com.sun.tools.javac.code.Symbol$CompletionFailure: class file for org.apache.thrift.TEnum not found
[total 508ms]