While formatting for user diagnostics used `Display` for all most cases,
some small amount of cases used `Debug` instead.  Until now, `Display`
and `Debug` yielded the same output for many types. However, with path
trimming, we want to show a shorter path for the user, these cases need
fixing.

The first use case of this detection of regression for trimmed paths
computation, that is in the case of rustc, which should be computed only
in case of errors or warnings.

Our current user of this method is deeply nested, being a side effect
from `Display` formatting on lots of rustc types. So taking only the
caller to the error message is not enough - we should collect the
traceback instead.

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.

`run_compiler` is used by clippy and other tools, which should not have
the trimmed paths feature enabled by default, until we see it works well
for them.

Would also be nice to rename `TimePassesCallbacks` however it's a
submodule change.