[NFC-ish] Eagerly create Functions in binary parser

[tlively]

In preparation for using IRBuilder in the binary parser, eagerly create
Functions when parsing the function section so that they are already
created once we parse the code section. IRBuilder will require the
functions to exist when parsing calls so it can figure out what type
each call should have, even when there is a call to a function whose
body has not been parsed yet.

NFC except that some error messages change to include the new empty
functions.

[kripken]

This seems like it could be confusing for users and even for us when debugging, to see these unreachables in the function bodies. How about storing nullptr for the body, and making Print handle those (by printing "placeholder" or such)?

[tlively]

I was originally going that route, but Print isn't the only place that would need to change. ModuleUtils would need to update the type collection code not to trip over the nullptr, too.

[kripken]

Wait, how can we do type collection before the entire module is ready?

[tlively]

Collecting the types is a prerequisite for printing the module. Even when the module is not complete, it's still in a consistent enough state that we can walk it and collect types.

[kripken]

I see. If it's only in those two places, perhaps it makes sense to do? I'd lean that way myself.