[Darwin][Driver] Don't pass a file as object_path_lto during ThinLTO

Summary:
After r327851, Driver::GetTemporaryPath will create the file rather than
just create a potientially unqine filename. If clang driver pass the
file as parameter as -object_path_lto, ld64 will pass it back to libLTO
as GeneratedObjectsDirectory, which is going to cause a LLVM ERROR if it
is not a directory.
Now during thinLTO, pass a temp directory path to linker instread.

rdar://problem/47194182

Reviewers: arphaman, dexonsmith

Reviewed By: arphaman

Subscribers: mehdi_amini, inglorion, jkorous, cfe-commits

Differential Revision: https://reviews.llvm.org/D56608

llvm-svn: 350970