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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197073 91177308-0d34-0410-b5e6-96231b3b80d8