I posted a longer description of the problem but I'm shortening it and combining my follow ups to here.
The problem that I'm having is that I've got an Xcode project that contains 1000+ files and has multiple targets which include two parallelized versions of my app--a console version and a GUI version. I'm using icc to compile my C code and llvm for objective C code. The build settings include llvm as the compiler, compile according to file type, and a build rule that uses icc to compile C source files. This setup has worked with every version of icc until XE13. With XE13U1 and Xcode 4.3.3 to 4.5.1, I get intermittent compilation errors. When everything does compile, I always get linking errors about missing symbols.
I've determined that the problem is that icc has trouble compiling multiple files concurrently. If I configure Xcode to compile only one file at a time, the compilation/linking problems go away. The problem is not with Xcode because I can switch back to XE12.1 and have no problem building my targets when compiling multiple files concurrently.
FWIW, I don't use precompiled headers when building with icc because I could never get work to work properly (it recompiles my prefix file for each file). I've never had issues when using pch with llvm or gcc.