Unfortunately, using __attribute__((constructor)) doesn't really work
because it races with all other constructors run pre-main. As a result
it's possible for a test's constructor to run after registration.
NSPrincipalClass gets instantiated only after all constructors have run.
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
* Use GoogleTest as a dependency of Firestore_Tests
* Remove top-level leveldb-library from HEADER_SEARCH_PATHS
* Add string_util_test to the project and get it to build
* Implement FSTGoogleTestTests, a bridge between GoogleTest and XCTest
Gil