[libFuzzer] Preserve position hint in auto dictionary

Currently, the position hint of an entry in the persistent auto
dictionary is fixed to 1. As a consequence, with a 50% chance, the entry
is applied right after the first byte of the input. As the position 1
does not appear to have any particular significance, this is likely a
bug that may have been caused by confusing the constructor parameter
with a success count.

This commit resolves the issue by preserving any existing position hint
or disabling the hint if the original entry didn't have one.

Reviewed By: morehouse

Differential Revision: https://reviews.llvm.org/D101686