Print the full func name (and path) in the debug MIR

[kkysen]

Print the full func name (and path) in the debug MIR.

The function name itself is not enough for readability. We can elide well-known items, such as things in the std prelude, but that's not what wasn't done, and I'm not sure how difficult it is to do that.

This reverts part of the changes in 7e31053, which I had missed (7e31053#r935146145).

That change changed things like this:

std::vec::Vec::<*mut i8>::push(move _29, move _30);

to this:

push(move _29, move _30);

which is IMO a lot less readable.

What we want is

Vec::<*mut i8>::push(move _29, move _30);

since Vec is in the std prelude.

Since I'm not sure how easy it is to do that prelude elision (I know rustc recently added this), this reverts those changes.

[kkysen]

I think a good alternative would be something like

Vec::push(move _29, move _30);

which is still fairly concise but a lot more easily readable.