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.
Recommitted as r197197 to reproduce the failure and reverted as r197199

Turns out there was unstable ordering in the type unit dumping code.
Fixed by using MapVector in DWARFContext to store the debug_types
comdat sections.

Recommitted as r197210 with a fix to dumping and reverted as r197211
because I was a bit gun shy and thought I saw a failure that turned out
to be unrelated.

So here we go - once more with feeling! \o/