* Add C++ build testing with clang and gcc

This adds Dockerfiles which test building flatc and the C++ library against clang
and gcc. See discussion at #5119.  It is derived from the Travis CI tooling.

The GRPC tests are failing due to #5099 so those are commented out.

These are run from the .travis.yml file rather than the tests/docker/languages
folder because the builds may each take longer than 30 minutes and were hitting
Travis timeouts.

Parallel builds and build caching attempt to keep the build times low.

* Add GCC 8.3 and Clang 7.0 with sanitizers into CI (based on #5130)

- Add a docker based on Debian Buster.
- Add C++ building scripts for the docker.
- Leak-sanitizer requires SYS_PTRACE.