[llvm-c-test] Fix unused variable warnings

This patch fixes:

  llvm/tools/llvm-c-test/debuginfo.c:211:12: error: unused variable
  'tag0' [-Werror,-Wunused-variable]

  llvm/tools/llvm-c-test/debuginfo.c:222:12: error: unused variable
  'tag1' [-Werror,-Wunused-variable]