This simplifies the import process back into google3 and allows us to add additional directories to the project without needing to update the project files for each directory we add. This shows up most clearly in the change to Firestore/Example/Firestore.xcodeproj/project.pbxproj: this no longer needs to list essentially every directory in the project as a header search path.
* Clang-format configuration for C++ files
* Add support C++-only sources to the podspec
* Podspec support for fully qualified imports
* xcodeproj changes for fully-qualified imports
* Use fully-qualified imports in Firestore sources
* Make all FIRLogger functions extern "C"
* Make FSTIntegrationTestCase Objective-C++
Keep external interface to FSTIntegrationTestCase Objective-C
* Add explicit dependency on Firebase/Community/Core from Tests
This fixes a linkage error on a missing _FIRSetLoggerLevel symbol
Paul Beusterien