DWARF v5 status in Clang/LLVM 7.0

Preliminary/experimental support for DWARF v5 is in Clang/LLVM 7.0.
If you compile with `-gdwarf-5 -O0` you should get debug information
that is fully compliant with DWARF v5, including the new .debug_names
accelerator tables.

Higher optimization levels will likely emit some debug-info sections
in DWARF v4 format; type units and split DWARF are known not to be
compliant. But we're getting there!

Many thanks to the contributors and reviewers for their time and
energy. Nearly every part of the DWARF 5 work has turned out to be
more complicated than expected, but we're definitely making progress!