TH: Abstract function for computing constructor names

We'll need this in another place shortly, so let's abstract it and add
proper verification of whether we were passed a type name correctly;
the previous version would have failed with a pattern match failure,
instead of an explicit message.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>