[dsymutil] Support more than 4 architectures

When running dsymutil on a fat binary, we use temporary files in a small
vector of size four. When processing more than 4 architectures, this
resulted in a user-after-move, because the temporary files got moved to
the heap. Instead of storing an optional temp file, we now use a unique
pointer, so the location of the actual temp file doesn't change.

We could test this by checking in 5 binaries for 5 different
architectures, but this seems wasteful, especially since the number of
elements in the small vector is arbitrary.

llvm-svn: 362621