Validate function imports

[kripken]

We validate functions in parallel, but function-parallel passes do not run on imports, so we did not issue a validation error on an import using a disallowed type, for example.

All the changes in visitFunction are just to group all the parts using body to the end, and putting them behind a check for body.

(diff without whitespace is smaller)

Found using #6310 #6312

[tlively]

function-parallel passes do not run on imports

I wonder where else we are messing this up...