According to the standard `DW_LNCT_directory_index` can be `data1`,
`data2`, or `udata` (see 6.2.4.1). The code was using `data1`, but this
limits the number of directories to 256, even if the variable holding
the directory index is a `uint64_t`. `dsymutil` was hitting an assertion
when trying to write directory indices higher than 255.
Modify the classic and the parallel DWARF linkers to use `udata` and
encode the directory indices as ULEB128 and provide a test that has more
than 256 directories to check the changes are working as expected.
For people that were using `dsymutil` with CUs that had between 128-256
directories, this will mean that for those indices 2 bytes will be used
now, instead of just one.
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