Emit forward decls for structs with declarations only when we are
limiting debug info.

FIXME: There is still work left to do here, the testcase should work even with -flimit-debug-info.

rdar://problem/14101097

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184252 91177308-0d34-0410-b5e6-96231b3b80d8