[lldb/Reproducers] Instrument SBFile

Currently SBFile isn't really instrumented, which was causing trouble
when capturing and replaying the Python test suite. The class is
particularly tricky because one of its constructors takes a FileSP which
isn't instrumented. Until we have proper shadowing in place, we'll
simply always record a nullptr.

Differential revision: https://reviews.llvm.org/D73992