[test] Add libLTO as a clang test dependency on Darwin

Running 'check-clang' on a stock checkout of llvm+clang doesn't work on
Darwin, because test/Driver/darwin-ld-lto.c can't find libLTO.dylib. Add
libLTO as a clang test dependency on Darwin to fix the problem.

Note: We don't have this issue with check-all because libLTO is in the
test-depends target.

Differential Revision: https://reviews.llvm.org/D24042

llvm-svn: 280142