[analyzer] Accept references to variables declared "extern void" (C only).

In C, 'void' is treated like any other incomplete type, and though it is
never completed, you can cast the address of a void-typed variable to do
something useful. (In C++ it's illegal to declare a variable with void type.)

Previously we asserted on this code; now we just treat it like any other
incomplete type.

And speaking of incomplete types, we don't know their extent. Actually
check that in TypedValueRegion::getExtent, though that's not being used
by any checkers that are on by default.