Omit the `note: ` prefix and the snippet from the "in import" note that
precedes a diagnostic.
This makes our diagnostic output more closely match that of Clang and
GCC.
On #3792 it was requested to put context-related diagnostic messages
prior to the diagnostic error. In order to support that, remove the
distinction that an error needs to come first on DiagnosticMessage.
I wanted to choose one or the other. I think some code has been using
each from early on. We're predominately using `filename`, so
consolidating on that. This conveniently matches the [Google dev doc
style guide](https://developers.google.com/style/word-list#filename)
(which we use for docs) which says "filename: Not file name".
In toolchain:
```
╚╡git grep file_name . | wc -l
35
╚╡git grep filename . | wc -l
489
```
Followup for #2490
The switch of DiagnosticMessage to have DiagnosticMessage means we don't need to use unique_ptr. This means that copy constructors are implicit again and don't need to be avoided, but per discussion still keeping with moves. Comments on HandleDiagnostic try to capture the use of moves there.
I'm keeping DiagnosticLevel at the top-level, and adding some checking that notes are actually Notes.
Also, adding MakeMessage to unify some of the logic (this has particularly been bugging me around format_fn, and I ran into it here because of the Diagnostic -> DiagnosticMessage change). The addition of NoTypeDeduction is intended to avoid some duplicative comments that'd been piling up.
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
Richard Smith
Jon Ross-Perkins
Jon Meow