* Down-integrate internal changes

* Update generated code for php, objc and csharp.

* Add missing dependency in conformance_php

Replace update_version shell script with a python script

Replace update_version shell script with a python script, which is much better at handling XML rewrite. Update the PHP version too. As part of rewrite, some XML file format will change a little, but the semantics is still the same so it should be okay.

ObjC compiler changes to allow for easier import

Use docker for csharp, java_compatibility, and python_compatibility.

Use docker for csharp, java_compatibility, and python_compatibility. Remove prepare_build_linux_rc that is no longer needed.

By putting the NSAssert behind a DEBUG flag we aren't copying filepaths
into release builds.

Updated Bazel workspace in examples/ to handle zlib dependency

* Create a script to help update the version automatically

* Update comment in the update_version script

* Update script to fix special handling for common.h and Makefile.am

* Revise update_version script to only update the other constants in common.h if it is a major release. Reset Makefile.am version to 17:0:0

To make this work, I created a symlink to the existing zlib.BUILD file.
Maybe there is a better way to do this, but this is at least a quick
workaround to fix the build.

Make Protobuf compatible with C# 6

* Add Bazel config for optional zlib support

* Add hard dependency on zlib

* Remove unused config_setting

Windows: install missing headers

Fix TimestampPartial operator == overload implementation: Cannot call == inside it, otherwise it would cause stack overflow

fix: cmake install

Just like fields, some extension fieldnames can be named such that they appear
to have meaning to ARC. Add the annotation to the compiler will get things
correct.

Add a bunch of extensions to allow inspection on generation to ensure things
are correct.

This should help the Swift compiler if these apis are used because it
changes the codegen to not need copies/retains.

This should help the Swift compiler if these apis are used because it
changes the codegen to not need copies/retains.

For deprecated fields, identify the deprecated field and source file in the deprecation message. For deprecated files, identify the deprecated file in deprecation messages of generated interfaces. This additional context in deprecation messages will help provide developers with more context which could help them seek recommended alternatives to deprecated interfaces.

* Add kokoro build for python source package

* Use libc++ for xcode 10 (#5303)

The xcode 10 removes the deprecated libstdc++ library. We could set
"MACOSX_DEPLOYMENT_TARGET" to "10.9" to use libc++ instead.

* Add python 3.7 build

* Add build for python 3.7 on linux and windows

* Remove unused source build

* Add comment

* Fix $MACOSX_DEPLOYMENT_TARGET mismatch

* Fix MACOSX_DEPLOYMENT_TARGET mismatch

* Add missing import for sysconfig

* Add missing imports

* adds string-to-int and int-to-string methods to enums

* remove check for valueToName property in EnumTrait

* Remove unused imports

* Update to avoid using EnumTrait

* Remove EnumTrait

* Update enum types

* Move name and value methods into generated classes

* Remove functions from GPBUtil

* Test well known enums

* Implement enum value to/from name in c extension

* Only generate use statement when namespace is present

Avoid octal constants in strutil.cc

In some cases proto files that want/need to use the objc_class_prefix option have
types that already have the prefix on a subset of their names. In this case we don't
want to duplicate the prefix.

Added tests for this (and prefixes in general).

The code now matches what the comment above it actually generates.

Also tweak that code so it actually compiles for me, and to avoid
embedding /* and */ in comments because that's generally a reasonable
thing to trigger warnings.