[Triton-MLIR] Enable libdevice for ptx backend when has external functions. (#848)

At the phase from ptx to cubin, check whether llvm::Module has external
functions. if has, link with libdevice at:
https://github.com/openai/triton/blob/triton-mlir/python/triton/language/libdevice.10.bc

include/triton/Target/LLVMIR/LLVMIRTranslation.h

@@ -1,5 +1,6 @@
 #ifndef TRITON_TARGET_LLVMIRTRANSLATION_H
 #define TRITON_TARGET_LLVMIRTRANSLATION_H
+#include "llvm/ADT/StringRef.h"
 #include <memory>
 #include <vector>
 
@@ -29,6 +30,8 @@ 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