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/call.cpp

@@ -52,6 +52,7 @@ static auto SplitCallArgumentList(Context& context,
 }
 
 auto PerformCallToCppFunction(Context& context, SemIR::LocId loc_id,
+                              SemIR::InstId callee_expr_id,
                               SemIR::CppOverloadSetId overload_set_id,
                               SemIR::InstId self_id,
                               llvm::ArrayRef<SemIR::InstId> arg_ids,
@@ -59,8 +60,9 @@ auto PerformCallToCppFunction(Context& context, SemIR::LocId loc_id,
   auto [template_arg_ids, function_arg_ids] =
       SplitCallArgumentList(context, arg_ids);
   auto callee_id = PerformCppOverloadResolution(
-      context, loc_id, context.cpp_overload_sets().Get(overload_set_id),
-      template_arg_ids, self_id, function_arg_ids);
+      context, loc_id, callee_expr_id,
+      context.cpp_overload_sets().Get(overload_set_id), template_arg_ids,
+      self_id, function_arg_ids);
   SemIR::Callee callee = GetCallee(context.sem_ir(), callee_id);
   CARBON_KIND_SWITCH(callee) {
     case CARBON_KIND(SemIR::CalleeError _): {