The rtti descriptor classes and associated variables are constructed as 
needed during compilation.  On the modules branch I found it best to 
write them by special reference and reconstruct them on read back.  That 
meant some changes to how rtti currently handles them.  This patch 
implements most of those changes, which are useful in their own right.

Internally we use an enumeration to identify the different 
__FOO_type_info classes, and create pseudo versions that don't escape 
the compilation.  These match definitions in the cxxabi.h header file. 
However, we were tangling up 'tell me the index for the typeinfo 
container for type X', with 'construct the typeinfo container for type 
X'.  This patch cleanly separates those questions (between which I can 
insert module streaming).

The typeinfo construction now lazily constructs all these descriptor 
types.  It removes a hole in the enumeration (there are no VMI 
descriptors with zero bases).  Also, when looking to see if we're 
building the runtime (needed to enable a piece of internal magic), we no 
longer use xref_tag, which will construct an incomplete type.  Instead 
use regular qualified lookup in the ABI namespace.

Applied to trunk.

nathan
-- 
Nathan Sidwell