* Replacing FIR_SWIFT_NAME macro with NS_SWIFT_NAME.
This pushes the minimum Xcode version to 7.3, as NS_SWIFT_NAME was
limited before that version (which is why the macro was introduced in
the first place).
* Fixed FIRMessaging header
* Changes "@import" to "@import" to make the code buildable when module is disabled.
Also rearranges code to merge two #if blocks.
* Makes comments more accurate.
* Fixes comments.