[libstdc++] New implementation of concept checking

[Jason Merrill]

>>>>> "Phil" == Phil Edwards <pedwards@disaster.jaj.com> writes:

> It Would Be Cool if we could tell the compiler, "oh, after instantiating
> this symbol, don't bother emitting it at all, not even as a weak symbol,"
> which would give us zero space cost as well as zero runtime cost.  Heh.

We shouldn't be emitting weak symbols that aren't referenced.

> 2)  Since large chunks of libstdc++ are built -fno-implicit-templates,
> the instantiations resulting from the typedefs become unresolved symbols
> during final executable linking.

So they are referenced, then.  That would seem to be the bug...

Jason