gn build: Follow-up to r364491 "[GN] Update build files"

- Merge r364427 (GSYM lib) more: It was missing the new unit test
  (as pointed out by llvm/utils/gn/build/sync_source_lists_from_cmake.py),
  and it had some superfluous deps not present in the cmake build.

- Merge r364474 (clang DependencyScanning lib) more: The deps didn't
  quite match cmake.

llvm-svn: 364501