Fix a crash when emitting dbeug info for forward-declared scoped enums.
It is possible for enums to be created as part of their own
declcontext. We need to cache a placeholder to avoid the type being
created twice before hitting the cache.

<rdar://problem/24493203>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@259975 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 6fbed68720badec2bcafeb98ecb554c681ae75ad)
2 files changed