[Reproducers] Pass FileCollector around as a shared_ptr (NFC)

Instead of passing the FileCollector around as a reference or raw
pointer, use a shared_ptr. This change's motivation is twofold. First it
adds compatibility for the newly added `FileCollectorFileSystem`.
Secondly, it addresses a lifetime issue we only see when LLDB is used
from Xcode, where a reference to the FileCollector outlives the
reproducer instance.

llvm-svn: 367258