firebase-ios-sdk

Rework CMake builds of Firebase Frameworks (#3499)

* Add sed scripts that find the versions of podspecs

... without fully loading those via Pod::Spec, which is slow.

* Avoid compiling with different visibility settings

... in different translation units. This removes warnings about linking
googletest causing weak symbols becoming strong.

* Add CMake objc_framework

* Define FirebaseCore via objc_framework

* Define FIRCoreDiagnosticsInterop via objc_framework

* Define GoogleUtilities with objc_framework

* Define FirebaseCoreDiagnosticsInterop via objc_framework

* Define FirebaseAuthInterop via objc_framework

* Don't duplicate Core's dependency information everywhere

* Remove podspec_framework

* Fix compilation error with -Wstrict-prototyes