Render diagnostics through the terminal library (#7683)

Diagnostic rendering was part of the data model: `Loc::FormatLocation`
and
`Loc::FormatSnippet` wrote plain text to a stream, so no capability,
color, or
terminal width could reach them.

A diagnostic is now one message with labels, contexts, and location
information
attached, and `Diagnostics::Renderer` draws it into a
`Terminal::Buffer`. The
design is in `toolchain/docs/diagnostics_rendering.md` (#7668),
authoring in
`toolchain/docs/diagnostics.md`.

Before:

```
example.carbon:6:3: error: 1 argument passed to function expecting 0 arguments
  Run0(1);
  ^~~~~~~
example.carbon:3:1: note: calling function declared here
fn Run0() {}
^~~~~~~~~~~
```

After:

```
error: 1 argument passed to function expecting 0 arguments
       ╭─┤ example.carbon:6:3
       ┆
     3 │ fn Run0() {}
       · ─────┬─────
       ·      ╰──┤ calling function declared here
       ┆
->   6 │   Run0(1);
       ·   ───────
       ╰────────────
```

`--no-diagnostic-snippets` gives a compact form, `example.carbon:6:3-9`.
File
tests use it; the rendering tests under `toolchain/diagnostics/testdata`
ask for
snippets.

C++ interop diagnostics are data now rather than text Clang drew:
`SemIR::ClangSourceLocId` names a `clang::SourceRange`, and a Clang
location is
marked as the point it names.

`StreamConsumer` moves to a `:renderer` target that `:emitter` does not
depend
on, keeping `common/terminal` out of `consumer.h`. Capabilities come
from the
file behind the error stream, so tests get plain ASCII.

`renderer_fuzzer` draws diagnostics built from fuzzer bytes with every
capability combination, since a shape the renderer doesn't expect must
never be
why the compiler dies while reporting a problem.

TODOs mark the follow-ups: line weights and crossing lines, wrapping
that keeps
a quoted type whole, and ordering diagnostics that share a byte offset.

Also: `IncorrectExtension`'s hint moves into its message, since its note
had no
location of its own, and `strip_noise` in `check_diagnostics.py` blanks
multi-line `#define`s instead of deleting them.

Assisted-by: Claude Code

toolchain/check/testdata/interop/cpp/function/import/pointer.carbon

@@ -43,13 +43,10 @@ fn G() -> const Cpp.NonNullableParamS;
 
 fn F() {
   var s: const Cpp.NonNullableParamS = G();
-  // CHECK:STDERR: fail_import_non_nullable_pointer_param_using_const_value.carbon:[[@LINE+8]]:32: error: no matching function for call to 'non_nullable_param_foo' [CppInteropParseError]
-  // CHECK:STDERR:    18 |   Cpp.non_nullable_param_foo(&s);
-  // CHECK:STDERR:       |                                ^
-  // CHECK:STDERR: fail_import_non_nullable_pointer_param_using_const_value.carbon:[[@LINE-9]]:10: in file included here [InCppInclude]
-  // CHECK:STDERR: ./non_nullable_pointer_param.h:4:6: note: candidate function not viable: 1st argument ('const NonNullableParamS * _Nonnull') would lose const qualifier [CppInteropParseNote]
-  // CHECK:STDERR:     4 | auto non_nullable_param_foo(NonNullableParamS* _Nonnull) -> void;
-  // CHECK:STDERR:       |      ^                      ~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_import_non_nullable_pointer_param_using_const_value.carbon:[[@LINE+5]]:3-28: error: no matching function for call to 'non_nullable_param_foo' [CppInteropParseError]
+  // CHECK:STDERR: included from: fail_import_non_nullable_pointer_param_using_const_value.carbon:[[@LINE-7]]:10 [InCppInclude]
+  // CHECK:STDERR: ./non_nullable_pointer_param.h:4:6: note: candidate function not viable [CppInteropParseNote]
+  // CHECK:STDERR: ./non_nullable_pointer_param.h:4:29-55: note: 1st argument ('const NonNullableParamS * _Nonnull') would lose const qualifier [CppInteropParseNoteReason]
   // CHECK:STDERR:
   Cpp.non_nullable_param_foo(&s);
 }
@@ -417,12 +414,8 @@ import Cpp library "deduced_pointer_param.h";
 
 fn F() {
   // This could return a null pointer, so should not be given the type `Cpp.DeducedPointerS*`.
-  // CHECK:STDERR: fail_import_deduced_pointer_param_as_optional_pointer.carbon:[[@LINE+7]]:40: error: cannot implicitly convert expression of type `Core.Optional(Cpp.DeducedPointerS* as Core.OptionalStorage)` to `Cpp.DeducedPointerS*` [ConversionFailure]
-  // CHECK:STDERR:   let unused a: Cpp.DeducedPointerS* = Cpp.Indirect({} as Cpp.DeducedPointerS);
-  // CHECK:STDERR:                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-  // CHECK:STDERR: fail_import_deduced_pointer_param_as_optional_pointer.carbon:[[@LINE+4]]:40: note: type `Core.Optional(Cpp.DeducedPointerS* as Core.OptionalStorage)` does not implement interface `Core.ImplicitAs(Cpp.DeducedPointerS*)` [MissingImplInMemberAccessInContext]
-  // CHECK:STDERR:   let unused a: Cpp.DeducedPointerS* = Cpp.Indirect({} as Cpp.DeducedPointerS);
-  // CHECK:STDERR:                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_import_deduced_pointer_param_as_optional_pointer.carbon:[[@LINE+3]]:40-78: error: cannot implicitly convert expression of type `Core.Optional(Cpp.DeducedPointerS* as Core.OptionalStorage)` to `Cpp.DeducedPointerS*` [ConversionFailure]
+  // CHECK:STDERR: fail_import_deduced_pointer_param_as_optional_pointer.carbon:[[@LINE+2]]:40-78: note: type `Core.Optional(Cpp.DeducedPointerS* as Core.OptionalStorage)` does not implement interface `Core.ImplicitAs(Cpp.DeducedPointerS*)` [MissingImplInMemberAccessInContext]
   // CHECK:STDERR:
   let unused a: Cpp.DeducedPointerS* = Cpp.Indirect({} as Cpp.DeducedPointerS);
 }