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/lower/function_context.cpp

@@ -7,6 +7,7 @@
 #include "common/pretty_stack_trace_function.h"
 #include "common/vlog.h"
 #include "toolchain/base/kind_switch.h"
+#include "toolchain/diagnostics/renderer.h"
 #include "toolchain/sem_ir/diagnostic_loc_converter.h"
 #include "toolchain/sem_ir/expr_info.h"
 #include "toolchain/sem_ir/file.h"
@@ -69,13 +70,13 @@ auto FunctionContext::LowerBlockContents(SemIR::InstBlockId block_id) -> void {
         &file_context_->context().tree_and_subtrees_getters(), &sem_ir());
     auto converted = converter.Convert(SemIR::LocId(inst_id_for_stack_trace),
                                        /*token_only=*/false);
-    converted.loc.FormatLocation(output);
+    output << Diagnostics::FormatLocation(converted.loc) << ": ";
     // TODO: Format SemIR for the instruction we were lowering?
     output << "Lowering "
            << sem_ir().insts().Get(inst_id_for_stack_trace).kind().ir_name()
            << "\n";
     // Crash output has a tab indent; try to indent slightly past that.
-    converted.loc.FormatSnippet(output, /*indent=*/10);
+    Diagnostics::PrintSnippet(output, converted.loc, /*indent=*/10);
   });
 
   for (auto inst_id : sem_ir().inst_blocks().Get(block_id)) {