DebugInfo: Move type units into the debug_types section with appropriate...

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.

llvm-svn: 197073