implement a nice new optimization: CodeGenTypes::UpdateCompletedType

is called whenever a tag type is completed.  We previously used that
as the sign to layout the codegen representation for the tag type,
which worked but meant that we laid out *every* completed type, whether
it was used or not.

Now we just lay out the type if we've already seen it somehow else.
This means that we lay out types we've used but haven't seen a body
for, but we don't lay out tons of stuff that noone cares about.

llvm-svn: 134866