`CodeSuggestion` doesn't live in the `diagnostic` module.

Fixes #38384

Most of the Rust community uses 4 spaces for indentation,
but there are also tab users of Rust (including myself!).

This patch fixes a bug in error printing which mispositions
error indicators when near code with tabs.

The code attempted to fix the issue by replacing spaces
with tabs, but it sadly wasn't enough, as sometimes
you may not print spaces but _ or ^ instead.

This patch employs the reverse strategy: it replaces each
tab with a space, so that the number of _ and ^ and spaces
in error indicators below the code snippet line up
perfectly.

In a study [1] preceeding this patch, we could see that
this strategy is also chosen by gcc version 6.3.0.

Its not perfect, as the output is not beautiful, but its
the easiest to implement. If anyone wants to improve on
this, be my guest! This patch is meant as improvement of
the status quo, not as perfect end status. It fixes the
actual issue of mispositioning error indicators.

[1]: https://github.com/rust-lang/rust/issues/38384#issuecomment-326813710

This commit introduces the ability to create and emit `Diagnostic`
structures from proc-macros, allowing for proc-macro authors to emit
warning, error, note, and help messages just like the compiler does.

This adds a function to `DiagnosticBuilder` to delay the entire diagnostic as a
bug to be emitted at a later time. This'll end up getting used in the compiler
in the subsequent commits...

Fixes #41701.

Fixes #44078. Fixes #44023.

The start_col member is given in chars,
while the code previously assumed it was given in bytes.

The more basic issue #44080 doesn't get fixed.

This commit alters the `rustc::ty::maps` implementation to ensure that all
output diagnostics from the compiler are tracked for the duration of each query.
These are then intended to be replayed back the first time a cached value is
loaded, and otherwise the cache should operate the same as it does today.

Closes #42513

The motivation here is to allow rustfmt to recover from parse errors
after failing to parse macros.

Like #43008 (f6689991), but _much more aggressive_.

Now there's a way to add suggestions that hide the suggested code when
presented inline, to avoid weird wording when short code snippets are
added at the end.

When there're more than one suggestions in the same diagnostic, they are
displayed in their own block, instead of inline. In order to reduce
confusion, those blocks now display the line number.

This reverts commit 5558c64f.

* The lazy loading mechanism has been moved to a more appropriate place.
* Return values from the functions invoked there are properly used.
* Documentation has gotten some minor improvements.
* Possibly some larger restructuring will need to take place still.