Make sure to mark the interface as completed when we see an

Make sure to mark the interface as completed when we see an
@implementation that closes a @class delcaration.
 - I don't know how to make a test case for this, but this strengthens
   the invariants that hold internally. The functionality change here
   is the edit to SemaDeclObjC.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@69728 91177308-0d34-0410-b5e6-96231b3b80d8