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/cpp/overload_resolution.cpp

@@ -263,7 +263,7 @@ auto ComputeClangDeclSignatureFromBestViableFunction(
 }
 
 auto PerformCppOverloadResolution(
-    Context& context, SemIR::LocId loc_id,
+    Context& context, SemIR::LocId loc_id, SemIR::InstId callee_expr_id,
     const SemIR::CppOverloadSet& overload_set,
     llvm::ArrayRef<SemIR::InstId> template_arg_ids, SemIR::InstId self_id,
     llvm::ArrayRef<SemIR::InstId> arg_ids) -> SemIR::InstId {
@@ -287,7 +287,13 @@ auto PerformCppOverloadResolution(
   }
   auto& arg_exprs = *maybe_arg_exprs;
 
-  clang::SourceLocation loc = GetCppLocation(context, loc_id);
+  // What a diagnostic about the call marks is the callee, not the call: the
+  // message names the callee, and the arguments it was given are described by
+  // the notes hanging off each candidate. This is where Clang points such a
+  // diagnostic too.
+  clang::CharSourceRange callee_range =
+      GetCppRange(context, SemIR::LocId(callee_expr_id));
+  clang::SourceLocation loc = callee_range.getBegin();
 
   // Add candidate functions from the name lookup.
   const auto& rewrite_info = overload_set.operator_rewrite_info;
@@ -332,7 +338,8 @@ auto PerformCppOverloadResolution(
       candidate_set.NoteCandidates(
           clang::PartialDiagnosticAt(
               loc, sema.PDiag(clang::diag::err_ovl_no_viable_function_in_call)
-                       << GetCppName(context, overload_set.name_id)),
+                       << GetCppName(context, overload_set.name_id)
+                       << callee_range),
           sema, clang::OCD_AllCandidates, arg_exprs);
       return SemIR::ErrorInst::InstId;
     }
@@ -340,15 +347,27 @@ auto PerformCppOverloadResolution(
       candidate_set.NoteCandidates(
           clang::PartialDiagnosticAt(
               loc, sema.PDiag(clang::diag::err_ovl_ambiguous_call)
-                       << GetCppName(context, overload_set.name_id)),
+                       << GetCppName(context, overload_set.name_id)
+                       << callee_range),
           sema, clang::OCD_AmbiguousCandidates, arg_exprs);
       return SemIR::ErrorInst::InstId;
     }
     case clang::OverloadingResult::OR_Deleted: {
-      sema.DiagnoseUseOfDeletedFunction(
-          loc, clang::SourceRange(loc, loc),
-          GetCppName(context, overload_set.name_id), candidate_set,
-          best_viable_fn->Function, arg_exprs);
+      // This is what `Sema::DiagnoseUseOfDeletedFunction` emits. That takes the
+      // callee's range as a `SourceRange`, whose end Clang would find by lexing
+      // the last Carbon token as C++.
+      clang::StringLiteral* deleted_message =
+          best_viable_fn->Function->getDeletedMessage();
+      candidate_set.NoteCandidates(
+          clang::PartialDiagnosticAt(
+              loc, sema.PDiag(clang::diag::err_ovl_deleted_call)
+                       << /*IsMember=*/false
+                       << GetCppName(context, overload_set.name_id)
+                       << (deleted_message != nullptr)
+                       << (deleted_message ? deleted_message->getString()
+                                           : llvm::StringRef())
+                       << callee_range),
+          sema, clang::OCD_AllCandidates, arg_exprs);
       return SemIR::ErrorInst::InstId;
     }
   }