Enums use varint, so if the value is negative it should end up being longer.

This was caught my new conformance test cases.

Some protos have enum values of "FOO" and "Foo", which the ObjC generation
then makes into the same thing. Just skip generating the enum element for
the duplicate as it would be a compile error because of the name collision.

The descriptors are still generated to support reflection and TextFormat
more completely.

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.

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).

We have code for converting C/C++/Objc keywords that appear in protos
to convert them so that they can be compiled.
One of the things we need to be careful of is accidentally overriding methods
that Apple declares in NSObject. It turns out that we have run into issues
where we conflict with "hidden" methods in NSObject or methods added by
categories. method_dump.sh collects all of the methods we care about for
macOS and iOS and dumps them into objectivec_nsobject_methods.h which
is then included in objectivec_helpers.cc as part of the build.

Added a pile of tests to verify that conversions are happening as expected.

* objectivec: Quash -Wself-assign

* objectivec: Set -Wno-vla when building

Objective-C protobuf uses VLAs for performance reasons. Ensure Clang
doesn’t complain about them.

Added new API to GPBEnumDescriptor to enable introspection of enum values.

Refactored implementation so that this contains a minimum of added code.

Clarified comments regarding behavior in the presence of the alias_allowed option.

Added unit tests for new functionality and for the alias case.

The tests can run as what Apple calls a Logic Test (under xctest), which means
it doesn't have to load an full UI App under the simulator, which speeds things
up a fair amount.

To ensure all headers aren't dependent on other things being imported
before/after them, make a source that just imports each header and add
it to the unittesting target, that way we ensure it can be included on
its own with ordering issues.

Also do this testing with a few generated headers that aren't part of
the library to help ensure the different generated imports needed are
complete.

Fixes #4246
Fixes #4247

Add tests to cover all the common special casing in the runtime code to
ensure things come out correctly.

Fix up warnings from Xcode 9.1

- Add a test to cover this and tweak the test to not use two merge
  paths to be sure things are as expected.

As announced: https://groups.google.com/forum/#!topic/protobuf/VX5qEmTW3y0

The ObjC side of https://github.com/google/protobuf/issues/272

Fixes https://github.com/google/protobuf/issues/3164.

Adds serial and parallel parsing tests to check if parallel parsing is faster than serial parsing, which it should

- Fix up -copyWithZone: to not leave the two registries sharing
  some of the storage by using -addExtensions:.
- Improve -addExtensions: to clone the sub dict when there is
  nothing to merge into.
- A ExtensionRegistry unittests.
- Update project schemes to not have extra things in perf scheme.

GPBMessageDropUnknownFieldsRecursively() is a new helper to drop the
unknownFields from a message and all sub messages (in fields or extensions).

- The Timestamp proto does not allow for negative nanos fields, so the seconds
  must be shifted and a positive nanos then applied.
- Tweak the helpers on Duration to make it clear there is no "base" time
  involved.
- Update the unittests for duration and timestamp to cover positive and
  negative NSTimeIntervals and what their impact is on the protos.

The Timestamp proto does not allow for negative nanos fields, so the seconds must be shifted and
a positive nanos then applied.

- If setting/clearing a repeated field/map that was objects, check the class
  before checking the autocreator.
- Just to be paranoid, don’t mutate within copy/mutableCopy for the autocreated
  classes to ensure there is less chance of issues if someone does something
  really crazy threading wise.
- Some more tests for the internal AutocreatedArray/AutocreatedDictionary
  classes to ensure things are working as expected.
- Add Xcode 8.2 to the full_mac_build.sh supported list.

- Check the parent file options for deprecation when deciding to tag Messages
  and Enums as deprecated.
- Within the generated source push/pop the warning for implementing deprecated
  things around a deprecated class implementation.
- Annotate the methods generated for extension fields as deprecated.
- Add a testing .proto file that covers deprecated fields, messages, enums,
  enum values and compile it into the unittests to confirm things compile
  cleanly.
- Add a testing .proto file that uses the file level option to make everything
  deprecated and compile it into the unittests to confirm things compile
  cleanly.

- Let Xcode 8 update settings on the projects/schemes.
- Migrate Swift tests to Swift 3 syntax.
- Update the build/test script:
  - Require Xcode 8 (because of the Swift 3 requirement for tests)
  - Update the devices to what Xcode 8 has (8.x simulator seem to fail even
    though they can be downloaded in Xcode 8)
- Update the travis images to ones with Xcode 8.