This is mainly useful for discontinuous functions because individual
parts of the function will have separate FDE entries, which can begin
many megabytes from the start of the function. However, I'm separating
it out, because it turns out we already have a test case for the
situation where the FDE does not begin exactly at the function boundary.
The test works mostly by accident because the FDE starts only one byte
after the beginning of the function so it doesn't really matter whether
one looks up the unwind row using the function or fde offset. In this
patch, I beef up the test to catch this problem more reliably.
To make this work I've also needed to change a couple of places which
that an unwind plan always has a row at offset zero.
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