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.
DWARF-4 introduced DWARF discriminators but both, the classic and the
parallel DWARF linkers discarded them so far.
After applying this patch dsymutil, which uses the DWARF linkers,
correctly preserves discriminator information.
This patch creates DWARFLinkerBase library, places DWARFLinker code into
DWARFLinker\Classic, places DWARFLinkerParallel into DWARFLinker\Parallel.
updates BOLT to use new library. This patch is NFC.
