Revert "DebugInfo: Remove explicit declaration-emissiong handling now that we have a more principled approach (the 'requires complete type' callback)"

This reverts commit r189013.

This is causing a segfault crash in a test case I have. Reverting while
I investigate the issue.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189906 91177308-0d34-0410-b5e6-96231b3b80d8