How to fix PCH error?
This is often a caching problem. Usually it can be resolved by holding down the Option key and choosing Product > Clean Build Folder...
Delete the DerivedData folder for the project. Look under Xcode preferences -> Locations to see where you save it.
It seems that the Product > Clean Build Folder... (with Option key pressed) works for most people. See the selected answer by @gaige.
If you're unlike most people (myself included) and this still causes you trouble XCode has likely left your shared precompiled headers elsewhere. For me they were in a folder similar to this:
/var/folders/<some random two characters>/<some random string>/C/com.apple.Xcode.502/SharedPrecompiledHeadersI just deleted the entire SharedPrecompiledHeaders folder and that solved things for me. I wouldn't go touching much else in here though...