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.

compiler/rustc_macros/src/query.rs

@@ -392,7 +392,7 @@ fn add_query_description_impl(
             #tcx: TyCtxt<'tcx>,
             #key: #arg,
         ) -> Cow<'static, str> {
-            format!(#desc).into()
+            ::rustc_middle::ty::print::with_no_trimmed_paths(|| format!(#desc).into())
         }
     };