[Modules][Objective-C] Use complete decl from module when diagnosing missing import

Summary:
Otherwise the definition (first found) for ObjCInterfaceDecl's might
precede the module one, which will eventually lead to crash, since
diagnoseMissingImport needs one coming from a module.

This behavior changed after Richard's r342018, which started to look
into the definition of ObjCInterfaceDecls.

rdar://problem/49237144

Reviewers: rsmith, arphaman

Subscribers: jkorous, dexonsmith, ributzka, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66982

llvm-svn: 372039