* 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
