[Backend] Rewrite code for linking external library to expose more inlining opportunities (#1037)

- Also make it cleaner. 
- And mark out the code needs to be fixed in `semantic.py`.

include/triton/Target/LLVMIR/LLVMIRTranslation.h

@@ -31,8 +31,6 @@ translateTritonGPUToLLVMIR(llvm::LLVMContext *llvmContext,
 std::unique_ptr<llvm::Module>
 translateLLVMToLLVMIR(llvm::LLVMContext *llvmContext, mlir::ModuleOp module);
 
-bool linkExternLib(llvm::Module &module, llvm::StringRef path);
-
 } // namespace triton
 } // namespace mlir