Fix LoadableByAddress to use @in_guaranteed for unowned values
LoadableByAddress was accidentally changing ownership of
direct_unowned values to @in (owned). This generates unsupported SIL
for on-stack partial applies, which now breaks SIL verification.
This also resulted in extra copies of values inside of closure
contexts. Before calling the original function, the value would need to
be copied onto the stack and the context would be destroyed. Now, we
simply pass a pointer directly from the closure context. See
IRGen/indirect_argument.sil+huge_partial_application.
I'm pretty sure fixing this has no effect on the mangling of public symbols.
