These files are already using C++ constructs, e.g. 'namespace', so
they're guaranteed to be compiled with a C++ compiler, not a C compiler.
It's only in the header files that '#ifdef __cplusplus' is needed.
PiperOrigin-RevId: 346804459
Change-Id: Ia49dc8e8f162a00f2dfbb77cf2fba84606f9dc1b