Run compilers from Java

It would be great if it was possible to run the compilers from Java, e.g. for use in Bazel persistent workers. Unfortunately, the only (somewhat) public method calls `System.exit`, making it unusable for persistent workers.

Start by locating the public compiler entry point described in the issue and tracing where it calls System.exit. Determine an API suitable for Java callers such as Bazel persistent workers; done means compilers can be invoked without terminating the hosting JVM.

Written by the indexing model from the issue text.