[C++0x] implementing forward declarations for enums

[Rodrigo Rivas]

> I had to initialize the variable nested_being_defined to get it to compile
> (possible uninitialized warning).  I initialized it to false.
Ok, actually it is never used uninitialized, but let's get rid of the warning.

> It looks like the first two are related.  What does an enum look like in
> tree?  Maybe the tree for enum could have a has_definition flag.
As far as I know it is a "tree_type". I was thinking of using
lang_flag_0 or similar, but I'm unsure if they are used for any other
purpose, or if there is a protocol to assign them...

Rodrigo