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.

- Use `.test` as the extension. Remove `.ll` from test extensions.
- The output generated by Python is the same, except for the checksum
  fields, which are constant, but does not seem to matter.