[MergeFunctions] Fix thunks for non-instruction debug info (#82080)

When MergeFunctions creates new thunk functions, it needs to copy over
the debug info format kind from the original function, otherwise we'll
mix debug info formats and run into assertions. This was exposed by a
downstream change that runs MergeFunctions before inlining, which caused
assertions when inlining attempted to inline thunks created by merging,
and the added test covers both scenarios where merging creates thunks.