[libcxxabi][test] Add missing return statement.
[libcxxabi][test] Add missing return statement.

This causes a test failure when running with -Werror=return-type.

NFC.

git-svn-id: 6a9f6578bdee8d959f0ed58970538b4ab6004734@271423 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi,llvm-mirror/libcxxabi