* Use dummy func when no lowered_funcs exists in Relay mod

* Dummy func -> CSourceModule with empty code str

* Added comments describing the empty CSouceModule

* Always import external modules w/o assertions

* Use CSourceModule as a fallback for LLVMModule

* Changed cond for target == llvm

* Create an empty LLVM module w/o using dummy func

* Avoid using IR str concat to create LLVM module

* Improved comments for codegen.LLVMModuleCreate

* Satisfy the linter for LLVMModuleCreate