Check for null ObjCInterfaceDecls returned from getClassInterface() when generating USRs.  While I have no test case for this (could not create one), this shows up in crash reports.  Tentatively fixes <rdar://problem/8452791>.
Check for null ObjCInterfaceDecls returned from getClassInterface() when generating USRs.  While I have no test case for this (could not create one), this shows up in crash reports.  Tentatively fixes <rdar://problem/8452791>.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@114392 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang