[IR] Keep a double break between functions when printing a module

This behavior appears to have changed unintentionally in
b0e97972.

Instead of printing the leading newline in printFunction, print it when
printing a module. This ensures that `OS << *Func` starts printing
immediately on the current line, but whole modules are printed nicely.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D73505