pretty: trim paths of unique symbols

If a symbol name can only be imported from one place for a type, and
as long as it was not glob-imported anywhere in the current crate, we
can trim its printed path and print only the name.

This has wide implications on error messages with types, for example,
shortening `std::vec::Vec` to just `Vec`, as long as there is no other
`Vec` importable anywhere.

This adds a new '-Z trim-diagnostic-paths=false' option to control this
feature.

On the good path, with no diagnosis printed, we should try to avoid
issuing this query, so we need to prevent trimmed_def_paths query on
several cases.

This change also relies on a previous commit that differentiates
between `Debug` and `Display` on various rustc types, where the latter
is trimmed and presented to the user and the former is not.

src/test/ui/consts/const-eval/const-eval-overflow-3b.stderr

@@ -10,7 +10,7 @@ error[E0277]: cannot add `u8` to `i8`
 LL |     = [0; (i8::MAX + 1u8) as usize];
    |                    ^ no implementation for `i8 + u8`
    |
-   = help: the trait `std::ops::Add<u8>` is not implemented for `i8`
+   = help: the trait `Add<u8>` is not implemented for `i8`
 
 error: aborting due to 2 previous errors