When a target has no C++ sources, but includes C++ object files
Xcode requires special treatment. A dummy source file is created
using the target's name to force the CMake Xcode generator to use
C++ as link language.

Append "_dummy" to the dummy source file's basename to avoid
collisions with the target's main source file when it
is target.cc.

Also adds a TODO to avoid adding the dummy source file when the
target being adjusted already has C++ sources.

Change-Id: I9795f6490e88eae6afc3a5901e6ee76a36cc801a