[modules] Maintain an AST invariant across module load/save: if any declaration
of a function has a resolved exception specification, then all declarations of
the function do.

We should probably improve the AST representation to make this implicit (perhaps
only store the exception specification on the canonical declaration), but this
fixes things for now.

The testcase for this (which used to assert) also exposes the actual bug I was
trying to reduce here: we sometimes fail to emit the body of an imported
special member function definition. Fix for that to follow.