cxxrtl: improve handling of FFs with async inputs (other than CLK).

cxxrtl: improve handling of FFs with async inputs (other than CLK).

Before this commit, the meaning of "sync def" included some flip-flop
cells but not others. There was no actual reason for this; it was
just poorly defined.

After this commit, a "sync def" means that a wire holds design state
because it is connected directly to a flip-flop output, and may never
be unbuffered. This is not affected by presence of async inputs.