Add testing of C++ with sanitizers (CI-Docker) (#5631)

* 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.

tests/docker/cpp_test.run.sh

@@ -0,0 +1,20 @@
+set -e
+
+JOBS=${JOBS:-1}
+export UBSAN_OPTIONS=halt_on_error=1
+export ASAN_OPTIONS=halt_on_error=1
+export MAKEFLAGS="-j$JOBS"
+
+config=$1
+echo ""
+echo "Build Flatbuffers project for '$config' with jobs=$JOBS"
+
+cmake . -DCMAKE_BUILD_TYPE=$config \
+  -DFLATBUFFERS_BUILD_TESTS=ON -DFLATBUFFERS_CODE_SANITIZE=ON
+cmake --build . --target all --clean-first -- -j$JOBS
+ctest --extra-verbose --output-on-failure -j$JOBS
+
+echo "Check generated code"
+.travis/check-generate-code.sh
+
+echo "C++ tests done"