Drop struct fields if the user destructor fails

[alexcrichton]

This commit changes drop glue generated for structs to use the invoke LLVM
instruction instead of call. What this means is that if the user destructor
triggers an unwinding, then the fields of the struct will still ge dropped.

This is not an attempt to support failing while failing, as that's mostly a
problem of runtime support. This is more of an issue of soundness in making sure
that destructors are appropriately run. The test included fails before this
commit, and only has one call to fail!(), yet it doesn't destroy its struct
fields.

[brson]

This may fix the original problem reported in #910 (though that issues has morphed into complaints about other things). Does run-fail/unwind-resource-fail2.rs work now?

[alexcrichton]

That test actually invokes fail!() twice, the first triggers unwinding and then while unwinding a second fail!() is triggered in the destructor, so this doesn't help dealing with that problem (sadly).