[Kaleidoscope][BuildingAJIT] Fix a bug in the symbol resolver in Chapter2.

Symbol resolution should be done on the top layer of the stack unless there's a
good reason to do otherwise. In this case it would have worked because
OptimizeLayer::addModuleSet eagerly passes all modules down to the
CompileLayer, meaning that searches in CompileLayer will find the definitions.
In later chapters where the top layer's addModuleSet isn't a pass-through, this
would break.

llvm-svn: 270899