[llvm-dwarfdump] --gdb-index: fix uninitialized TuListOffset

The test only checks the existence of the `Types CU list` line.
Unfortunately I can't make a better test because
{gcc,clang} -fuse-ld={lld,gold} --gdb-index do not give me a non-empty types CU list.

Reviewed By: ikudrin

Differential Revision: https://reviews.llvm.org/D63537

llvm-svn: 363800