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_middle/src/query/mod.rs

@@ -1255,6 +1255,11 @@ rustc_queries! {
             storage(ArenaCacheSelector<'tcx>)
             desc { "calculating the visible parent map" }
         }
+        query trimmed_def_paths(_: CrateNum)
+            -> FxHashMap<DefId, Symbol> {
+            storage(ArenaCacheSelector<'tcx>)
+            desc { "calculating trimmed def paths" }
+        }
         query missing_extern_crate_item(_: CrateNum) -> bool {
             eval_always
             desc { "seeing if we're missing an `extern crate` item for this crate" }