Updated the change log with changes for 3.6.0

Cherry-picked XCode 10 fixes into 3.6.x branch

* Update code to work for Xcode 10b

* Update README and test scripts to mention that Xcode 7 is no longer supported

Require C++11 and pass -std=c++11

This commit updates the Autotools build to require a C++11 compiler and
explicitly pass -std=c++11. This seems to be necessary for enabling
C++11 support in older compilers.

Cherry-pick changes into 3.6.x

These methods are used by grpc

Removed duplicate using statement from ReflectionUtil.cs

Fix php memory leak test (#4692)

Fixed JS parsing of unspecified map keys

We need to use a default of 0 when parsing unspecified map keys, instead
of failing an assertion.

This change was written by Michael Aaron (michaelaaron@google.com) but I
am cherry-picking it directly instead of waiting for the next sync of
Google-internal changes.

Cherry-pick php generated code changes into 3.6.x

In old generated code, the constructor of message doesn't provide a
default null to parent's constructor. Previously, in c extesnion, this
case was not handled.

* uses namespaces for nested messages and enums

* fixes namespaces for PHP dist

* fixes namespace for Descriptors, adds Cardinality and Kind

* fixes nested namespaces for reserved words and adds tests

* adds tests and generator fix for php class prefixes

* fixes escaping of protobuf packages, enum comments, misc others

* nice refactor of generated code

* adds class files for backwards compatibility

* simplifies code with templates

* adds compatibility files to makefile

* cleanup of generator and fixes nested namespace bug

* regenerates proto types

* remove internal BC classes

* adds deprecated warning, adds methods back

* simplifies if statement

* fixes dist files

* addresses review comments

* adds back TYPE_URL_PREFIX constant

* adds @deprecated to old nested class files

* skips tests which require a separate process when protobuf.so is enabled

* Adds tests for legacy nested classes that do not require separate processes to test

* uses legacy names for GPBUtil message check

* adds block for IDE @deprecated message

* Namespace for nested message/enum in c extension

* Remove unused code

* PHP array constructors for protobuf messages

* removes Descriptor from error message

* allows mergeFrom to accept an array

* only use initWithDescriptor if instanceof MapEntry

* adds doc comments

* removes ability for constructors to take arrays for submessages

* Revert "allows mergeFrom to accept an array"

This reverts commit b7b72182d561634af12c5c5c56a7cda3b33241f9.

* makes mergeFromArray protected and fixes mergeFrom whitespace

* Separates merging from JSON and merging from PHP array

* removes well-known types and json keys from array construction

* Addresses PR review comments

* cleans up tests

* fixes exception messages

* Adopt ruby_package in ruby generated code.

* Add test for ruby_package

* Adopt php_metadata_namespace in php code generator

The php_metadata_namespace is corresponded to the relative directory of
the metadata file. e.g., previously, the metadata file of foo.proto was
GPBMetadata/Foo.php. If the php_metadata_namespace is "Metadata\\Bar",
the metadata file will be Metadata/Bar/Foo.php.

* Handle empty php_metadata_namespace

Make sure to delete temporary maps used by FileDescriptorTables

I ran the tcmalloc heap checker in draconian mode, and it reported some
heap allocations that were still reachable when protobuf-test exited. To
fix that, I updated FileDescriptorTables to make sure its temporary
heap-allocated maps are deleted when it is destroyed.

Update version number to 3.6.0

Add .proto files to extract_includes.bat

* Throw error if user want to access message properties

* Fix typo

* Avoid direct check of class name

So that subclasses are also ok

* Use type hint in function signature.

use brew instead of easy_install in OSX (easy_install keep failing for a month)

Fix error in Clang UndefinedBehaviorSanitizer

Pointer Arguments to memcpy can not be null in UndefinedBehaviorSanitizer.
In this case, both the memory and the size was zero. This change allows
protoc to run under UndefinedBehaviorSanitizer.

Add mention to the "Build" section of the ReadMe of the requirement of installing an optional component to VS2017.