[MC/Dwarf] Unclamp DWARF linetables format on Darwin.

In r319995, we fixed the line table format to version 2 on Darwin
because dsymutil didn't yet understand the new format which caused test
failures for the LLDB bots. This has been resolved in the meantime so
there's no reason to keep this limitation.

rdar://problem/35968332

llvm-svn: 342136