[llvm-c-test] Fix warnings

This patch fixes:

  llvm/tools/llvm-c-test/debuginfo.c:254:20: error: unused variable
  'AddDbgRecordLast' [-Werror,-Wunused-variable]

  llvm/tools/llvm-c-test/debuginfo.c:257:20: error: unused variable
  'AddDbgRecordOverTheRange' [-Werror,-Wunused-variable]

  llvm/tools/llvm-c-test/debuginfo.c:264:20: error: unused variable
  'AddDbgRecordUnderTheRange' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/tools/llvm-c-test/debuginfo.c

@@ -253,17 +253,20 @@ int llvm_test_dibuilder(void) {
   assert(AddDbgRecordSecond != NULL);
   LLVMDbgRecordRef AddDbgRecordLast = LLVMGetLastDbgRecord(Add);
   assert(AddDbgRecordLast != NULL);
+  (void)AddDbgRecordLast;
   assert(AddDbgRecordSecond == AddDbgRecordLast);
   LLVMDbgRecordRef AddDbgRecordOverTheRange =
       LLVMGetNextDbgRecord(AddDbgRecordSecond);
   assert(AddDbgRecordOverTheRange == NULL);
+  (void)AddDbgRecordOverTheRange;
   LLVMDbgRecordRef AddDbgRecordFirstPrev =
       LLVMGetPreviousDbgRecord(AddDbgRecordSecond);
   assert(AddDbgRecordFirstPrev != NULL);
   assert(AddDbgRecordFirst == AddDbgRecordFirstPrev);
   LLVMDbgRecordRef AddDbgRecordUnderTheRange =
       LLVMGetPreviousDbgRecord(AddDbgRecordFirstPrev);
   assert(AddDbgRecordUnderTheRange == NULL);
+  (void)AddDbgRecordUnderTheRange;
 
   char *MStr = LLVMPrintModuleToString(M);
   puts(MStr);