This reverts commit 6d96493b.

* Compiler changes (extensions)

* Generated changes (extensions)

* Library changes (extensions)

* Adjusted a summary to indicate ContainingType can be null for extensions

* Compiler changes (custom option review + access level review)

* Generated code changes (custom options + access review)

* Library changes (custom options + access review)

* Support C# 6 with library changes

* Access HasValue by property

* Set access level of all extension classes to internal (revert in next PR)

* Added null checks to custom options

* Rebase on master and regenerate Conformance

* Removed second dictionary from ExtensionSet

* Rebased compiler changes

* Rebased generated code changes

* Rebased library changes + review changes

* Add more safety checks to extension accessors

* Remove instances where extension sets were unnecessarily allocated

* Remove cleared items from sets
Empty sets are now made null
IExtensionMessage -> IExtendableMessage

* Remove dead code from IExtensionValue impls

* Clean both repeated and single value extensions

* Add GetOrRegister method for repeated fields and allow clearing repeated extensions

* Add type safe ClearExtension methods, remove non-generic IExtendableMessage interface.

* Simplify ExtensionSet.TryMergeFieldFrom

* Rebase on master to resolve conflicts

* Fix Makefile.am

* Add ObjectIntPair to Makefile.am

* Update version number to 3.7.1

* Update version number in port_def.inc

also add useful Span-based methods for ByteString

* Add conformance test for nested listvalue

* Fix upb for parsing repeated Value/ListValue

* Add failed repeated ListValue conformance test into php failure list

* Bump target frameworks from netcoreapp1.0 to netcoreapp2.2.
Move global.json up to root of repo, change SDK ver to 2.2.100
Change .net core sdk in dockerfile for kokoro to ver 2.2.100

* Re-add curl install

* Change all exe target to 2.1

* Fix incorrect versions in global.json and Dockerfile

* Downgrade version to 2.1 to match exe targets

* introduce separate testing Dockerfile for C#

* revert changes to the shared Dockerfile

* use netcoreapp2.1 for C# conformance tests

* use language specific dockerfile for testing C#

* Edit compatibility tests script to use parameters instead of file copies

* install dotnet SDK on windows before running the tests

* update csharp_EXTRA_DIST

* Improve C# serialization performance of repeated fields for primitives.

* Changes based on feedback.

* Change compatibility tests to chec float, bool and double are fixed

* Changes based on feedback.

* In the compute methods use the newly created constants

* Update version number to 3.7.0

* Add php/releash.sh to Makefile.am

* Make cpp_distcheck exit on fail

* Fix #5513

* Added tests for invalid lengths when reading strings and bytes.
Added test for reading tags with invalid wire types in unknown field set.
Changed invalid length check in ReadString to match the one in ReadBytes

Signed-off-by: Xiang Dai <764524258@qq.com>

* Update version to 3.7.0

* Update version number for java bom

* Modify how end tags are encounted in merge code (compiler)

* Modify how end tags are encounted in merge code (generated)

* Modify how end tags are encounted in merge code (library)

* Regenerate generated code through generate_descriptor_proto.sh

* Modify how end tags are encounted in merge code (compiler)

* Modify how end tags are encounted in merge code (generated)

* Modify how end tags are encounted in merge code (library)

* Regenerate generated code through generate_descriptor_proto.sh

* Down-integrate internal changes to github.

* Fix BUILD file and regenerate csharp descritpor

* Down-integrate internal changes to github.

* fix csharp conformance test

* add comments in conformance.proto for text format

* fix comments

* fix comments, re-generated csharp file

* fix comments, re-generated csharp file

Even though the comments were indented to appear to go with the jspb
case/field, protoc doesn't collect comments like that, so these "hanging"
comments actually "attach" to the next thing added to each.  Looking at
https://github.com/protocolbuffers/protobuf/pull/5566 you see where
the generated code picked up the comment on the wrong field.

* Down-integrate internal changes to github.

* fix python conformance test

* fix csharp conformance test

* add back java map_lite_test.proto's optimize for option

* fix php conformance test