Image section headers are stored in the DBI stream, but we
had no way to dump them. This patch adds dumping support,
along with some tests that LLD actually dumps them correctly.
Differential Revision: https://reviews.llvm.org/D36332
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310107 91177308-0d34-0410-b5e6-96231b3b80d8
