DebugInfo: Move type units into the debug_types section with appropriate comdat grouping and type unit headers

This commit does not complete the type units feature - there are issues
around fission support (skeletal type units, pubtypes/pubnames) and
hashing of some types including those containing references to types in
other type units.

Originally committed as r197073 and reverted in r197079.

This commit originally got jumbled up with another build-breaking commit
and I can't find the failures I thought this caused anymore.
Recommitting to hopefully get some clean buildbot results to work from.
I have a sneaking suspicion there's unstable output in the comdat group
output of MCStreamer...