OffsetTypeInfo

[dsimcha]

According to this post:
http://www.digitalmars.com/pnews/read.php?server=news.digitalmars.com&group=digitalmars.D&artnum=86918

OffsetTypeInfo can be enabled when building LDC.  Why does it not appear to
work in current versions of DMD?  Is there a good reason why it's apparently
disabled?  The reason I ask is because with OffsetTypeInfo enabled, one could
almost trivially build a mostly-precise GC.  All that would be needed is for
each block to store a pointer to typeinfo.