* 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.

* Support for python grpc

* add few fixes

* Fixes build

* Fix python generator

* Add tests

* Fix grpc python test

* Fix tests and add incomplete python generator

* Fix python generator

* Add python generator methods

* Fix Appveyor build

* grpc python support v0.1

* Update tests

* update grpctest

* Remove duplicated code and fix a brace

* tests for flatbuffers grpc python

* Updated tests + removed SerializeToString, From String

* remove pickle import

* include missing files in ci - BUILD and generated test result

* Added basic schema evolution tests

* Add BUILD targets for evolution tests. Added to test/generate_code scripts

* Use vector.front() instead of vector.data()

* Added --scoped-enums option for evolution test

* Cloned idl_gen_general.cpp to idl_gen_csharp.cpp and removed java references

* Java generator changes

See https://github.com/servo/rust-smallvec/pull/175 for changelog.

Change-Id: Ia439bcc61bac5788792244d10e573b1fba54b347

* Automatic refractor of C++ headers to Google C++ style guide

* Automatic refractor of C++ source to Google C++ style guide

* Automatic refractor of C++ tests to Google C++ style guide

* Fixed clang-format issues by running clang-format twice to correct itself. Kotlin was missing clang-format on after turning it off, so it was changed,

Change-Id: Iccae8fe9409adbf3cd3013a5cf3368e068175ad3

Now identical to https://www.apache.org/licenses/LICENSE-2.0.txt

Change-Id: I6eab8bdee2472107dfc72265e204fdd16f3abc2e

Older clang versions raise:
```
./flatbuffers/base.h:365:1: error: unknown attribute 'no_sanitize' ignored [-Werror,-Wattributes]
__supress_ubsan__("alignment")
^
./flatbuffers/base.h:246:50: note: expanded from macro '__supress_ubsan__'
  #define __supress_ubsan__(type) __attribute__((no_sanitize(type)))
                                                 ^
```

Comparing https://releases.llvm.org/3.6.0/tools/clang/docs/AttributeReference.html
with https://releases.llvm.org/3.7.0/tools/clang/docs/AttributeReference.html
shows that __attribute__((no_sanitize(type))) is available since 3.7.0

* Skip generation of mutable union types

* Removed C# and Java unit tests that mutated a Union type

Fixes following clang -Wdocumentation warning:
```
flatbuffers.h:1762:17: error: parameter ']' not found in the function declaration [-Werror,-Wdocumentation]
  /// @param[in]] v A const reference to the `std::vector` of structs to
```

Kolin uses java library as dependency, which changed the way it access union vector recently
(e365c502).

This changes updates kotlin code generation to match Java's changes.

The condition was unnecessary and Detected by

PVS-Studio
V560 [CWE-571] A part of conditional expression is always true: !opts.use_flexbuffers. flatc.cpp 438

* start

* works for current usages!

* unpack: vector of struct

* optimize byte slice

* support nested struct

* support null table

* support struct

* support union

* update generated code

* grumble

* fix compiler warning

* update generated code

* wrap type in namespace

* bug

* wrap in namespace

* enum byte arrays

* generate struct for unions

* basic testing

* remove branching

* fix assert

* pack vector of fixed structs correctly

* omit null vectors

* Refactor Union Pack and UnPack methods

Remove append usage to increase code efficiency when dealing with large vectors

* generate goldens

* Rust: Add idiomatic iterator for Vector type

* Add comments explaining some implementation details

Found by the https://github.com/OSGeo/gdal/blob/master/gdal/scripts/fix_typos.sh
script on the internal copy of flatbuffers inside GDAL

* Rust: Fix Copy and Clone impls for a few generic types

* Add tests for Copy+Clone

* Wrap Copy+Clone checks in a #[test] function

- `--clean-first` runs `clean` target to remove previously generated files
- missed `monster_extra_generated.h` file added to the `flattests` target

- remove not necessary file `monster_test_generated.h` from sample_bfbs.cpp

* Avoid intentional unsigned integer overflow getting caught by sanitizers

* Adapt from suggested compiler compatible solution

* And a third way :)

* Add forceDefaults opt to python Builder

* Add test functions for force_default option for python builder

* Simplify

* Add force default test for UOffsetTFlags

* Annotate getters with @Pure when --java-checkerframework is specified.

Together with @Nullable, this allows users to use static analysis tools
like CheckerFramework to catch NPEs caused by unset fields.

* Don't annotate vector-of-tables item getters with @Nullable.

Since Flatbuffers don't support null items in vectors of tables.

Change-Id: I6fd6283b616c7a39bb878b1610e4ddf6e208fa0a

* byte buffer factory returned buffer is used instead of the requested capacity

* byte buffer factory returned buffer is used instead of the requested capacity

* Comment fix