fix MIR fn-ptr pretty-printing
An uninitialized function pointer would get printed as `{{uninit fn()}` (notice the unbalanced parentheses), and a dangling fn ptr would ICE. This fixes both of that.
However, I have no idea how to add tests for this.
Also, I don't understand this MIR pretty-printing code. Somehow the print function `pretty_print_const_scalar` actually *returns* a transformed form of the const (but there is no doc comment explaining what is being returned); some match arms do `p!` while others do `self =`, and there's a wild mixture of `p!` and `write!`... all very mysterious and confusing.^^
r? ``@oli-obk``
2d39300e2f