[Sema] Fix assumption about typo corrections containing no decl.
[Sema] Fix assumption about typo corrections containing no decl.

This can happen when the typo correction is coming from an external sema
source. Test case will follow in clang-tools-extra.

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@292927 91177308-0d34-0410-b5e6-96231b3b80d8

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