Haven't been able to make a repo case, but this should "fix" the problem
by avoid it completely.

- Move readOnlySemaphore_ into the .m file so it isn't exposed in any
  header.
- Move GPBGetObjectIvarWithField() also to go with the new limited
  visibility on the readOnlySemaphore_.

The generated code for enums needs atomics support, so generate the
import instead of relying on it via transitive imports. This will
make future changes to this likely likely to break generated code
and runtime support are mixed.

Followup to https://github.com/google/protobuf/pull/4184.

Followup to https://github.com/google/protobuf/pull/4184, keep the
import to not break any existing generated code that isn't regenerated
when they update to the newer protobuf code.

* Migrate away from deprecated OSAtomic APIs.

Apple recently updated the docs on dispatch_once to point out
that the storage for the dispatch_once_t must be static or global,
but not something that was ever used before as the implementation
doesn't use a memory barrier.  So we drop the use and create the
semaphore when needed and use an atomic swap deal with any
threading races.

Disable static analyzer for message semaphore creation

Exposes the currently registered extensions for a message and removes the internal sortedExtensionsInUse

Working on https://github.com/google/protobuf/issues/1599, specifically:
- Turn on more warnings that the Xcode UI calls out with individual controls.
- Manually add:
  -Wundef
  -Wswitch-enum
- Manually add and then diable in the unittests because of XCTest's headers:
  -Wreserved-id-macro
  -Wdocumentation-unknown-command
- Manually add -Wdirect-ivar-access, but disable it for the unittests and in
  the library code (via #pragmas to suppress it). This is done so proto users
  can enable the warning.

This will lower the amount of dispatch_semaphores created per Message when the
full object tree isn't walked in a way that would require them to be created.
Uses a dispatch_once_t for one time init of the dispatch_semaphore.

Apple engineers have pointed out that OSSpinLocks are vulnerable to live locking
on iOS in cases of priority inversion:
. http://mjtsai.com/blog/2015/12/16/osspinlock-is-unsafe/
. https://lists.swift.org/pipermail/swift-dev/Week-of-Mon-20151214/000372.html

- Use a dispatch_semaphore_t within the extension registry.
- Use a dispatch_semaphore_t for protecting autocreation within messages.
- Drop the custom/internal GPBString class since we don't have really good
  numbers to judge the locking replacements and it isn't required. We can
  always bring it back with real data in the future.

- Add more to the ObjC dir readme.
- Merge the ExtensionField and ExtensionDescriptor to reduce overhead.
- Fix an initialization race.
- Clean up the Xcode schemes.
- Remove the class/enum filter.
- Remove some forced inline that were bloating things without proof of performance wins.
- Rename some internal types to avoid conflicts with the well know types protos.
- Drop the use of ApplyFunctions to the compiler/optimizer can do what it wants.
- Better document some possible future improvements.
- Add missing support for parsing repeated primitive fields in packed or unpacked forms.
- Improve -hash.
- Add *Count for repeated and map<> fields to avoid auto create when checking for them being set.

- Style fixups in the code.
- map<> serialization fixes and more tests.
- Autocreation of map<> fields (to match repeated fields).
- @@protoc_insertion_point(global_scope|imports).
- Fixup proto2 syntax extension support.
- Move all startup code to +initialize so it happen on class usage and not app startup.
- Have generated headers use forward declarations and move imports into generated code, reduces what is need at compile time to speed up compiled and avoid pointless rippling of rebuilds.