This could happen if some really strange syntax is included in a source file, but since its in core libraries, its much more likely that something about the build files were corrupted.

Try (in order from least to most invasive/disruptive):

• do a clean build: Product > Clean
• clean out build artifacts: Window > Organiser then delete Derived Data.
• regenerate the whole project

If you do wind up having to regenerate the whole project, and you have actually coded changes that you want to keep, you can make a parallel project and drag and drop the stuff from the old project in to the new one.

Here’s some other tips to avoid file corruption:

• When you choose a directory to save the project to make sure its on
• your main MacintoshHD drive, and not on some external or secondary
• drive, or networked drive.
• Don’t use a drive that is subject to DropBox, shared to VMWare or 
• otherwise able to be modified on-the-fly by other processes/software.
• Turn off any anti-virus software, on access scanners and the like.

I’m not saying that these things absolutely will cause file corruption, but these are the usual suspects.

Here’s a screen shot of deleting Derived Data.