I found something similar in an old project I have today. What happened is that there was a subproject in my project and that one wasn't set to llvm5.
You may want to do a search in your directory to find a file specifying llvm 4.2, like such:
MyMac:~/path/to/my/project $ grep -rHin llvm *
MyProject.xcodeproj/project.pbxproj:334: GCC_VERSION = com.apple.compilers.llvmgcc42;
MyProject.xcodeproj/project.pbxproj:348: GCC_VERSION = com.apple.compilers.llvmgcc42;
If that's the case, try updating that subproject or removing it altogether, see if you can at least clean your build.
Hope this helps.