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/function.carbon

@@ -37,9 +37,7 @@ library "[[@TEST_NAME]]";
 import Cpp library "global.h";
 
 fn MyF() {
-  // CHECK:STDERR: fail_import_global_use_different_name.carbon:[[@LINE+4]]:3: error: member name `bar` not found in `Cpp` [MemberNameNotFoundInInstScope]
-  // CHECK:STDERR:   Cpp.bar();
-  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR: fail_import_global_use_different_name.carbon:[[@LINE+2]]:3-9: error: member name `bar` not found in `Cpp` [MemberNameNotFoundInInstScope]
   // CHECK:STDERR:
   Cpp.bar();
 }
@@ -59,9 +57,7 @@ library "[[@TEST_NAME]]";
 import Cpp library "special_name.h";
 
 fn MyF() {
-  // CHECK:STDERR: fail_import_special_name_call_unescaped.carbon:[[@LINE+4]]:3: error: member name `base` not found in `Cpp` [MemberNameNotFoundInInstScope]
-  // CHECK:STDERR:   Cpp.base();
-  // CHECK:STDERR:   ^~~~~~~~
+  // CHECK:STDERR: fail_import_special_name_call_unescaped.carbon:[[@LINE+2]]:3-10: error: member name `base` not found in `Cpp` [MemberNameNotFoundInInstScope]
   // CHECK:STDERR:
   Cpp.base();
 }
@@ -85,13 +81,10 @@ library "[[@TEST_NAME]]";
 import Cpp library "special_name.h";
 
 fn MyF() {
-  // CHECK:STDERR: fail_import_special_name_call_escaped_no_match.carbon:[[@LINE+8]]:15: error: no matching function for call to 'base' [CppInteropParseError]
-  // CHECK:STDERR:    15 |   Cpp.r#base(1);
-  // CHECK:STDERR:       |               ^
-  // CHECK:STDERR: fail_import_special_name_call_escaped_no_match.carbon:[[@LINE-6]]:10: in file included here [InCppInclude]
-  // CHECK:STDERR: ./special_name.h:2:6: note: candidate function not viable: requires 0 arguments, but 1 was provided [CppInteropParseNote]
-  // CHECK:STDERR:     2 | auto base() -> void;
-  // CHECK:STDERR:       |      ^
+  // CHECK:STDERR: fail_import_special_name_call_escaped_no_match.carbon:[[@LINE+5]]:3-12: error: no matching function for call to 'base' [CppInteropParseError]
+  // CHECK:STDERR: included from: fail_import_special_name_call_escaped_no_match.carbon:[[@LINE-4]]:10 [InCppInclude]
+  // CHECK:STDERR: ./special_name.h:2:6: note: candidate function not viable [CppInteropParseNote]
+  // CHECK:STDERR: ./special_name.h:2:10-11: note: requires 0 arguments, but 1 was provided [CppInteropParseNoteReason]
   // CHECK:STDERR:
   Cpp.r#base(1);
 }
@@ -133,9 +126,7 @@ import Cpp library "variadic.h";
 
 fn F() {
   //@dump-sem-ir-begin
-  // CHECK:STDERR: fail_todo_import_variadic.carbon:[[@LINE+4]]:3: error: semantics TODO: `Unsupported: Variadic function` [SemanticsTodo]
-  // CHECK:STDERR:   Cpp.variadic_foo(8);
-  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_todo_import_variadic.carbon:[[@LINE+2]]:3-21: error: semantics TODO: `Unsupported: Variadic function` [SemanticsTodo]
   // CHECK:STDERR:
   Cpp.variadic_foo(8);
   //@dump-sem-ir-end
@@ -148,9 +139,7 @@ fn F() {
 // --- static.h
 
 // TODO: Promote this warning to an error by default.
-// CHECK:STDERR: ./static.h:[[@LINE+3]]:13: warning: function 'static_foo' has internal linkage but is not defined [CppInteropParseWarning]
-// CHECK:STDERR:     6 | static auto static_foo() -> void;
-// CHECK:STDERR:       |             ^
+// CHECK:STDERR: ./static.h:[[@LINE+1]]:13: warning: function 'static_foo' has internal linkage but is not defined [CppInteropParseWarning]
 static auto static_foo() -> void;
 
 // --- todo_fail_import_static.carbon
@@ -161,9 +150,7 @@ import Cpp library "static.h";
 
 fn F() {
   //@dump-sem-ir-begin
-  // CHECK:STDERR: todo_fail_import_static.carbon:[[@LINE+4]]:18: note: used here [CppInteropParseNote]
-  // CHECK:STDERR:    12 |   Cpp.static_foo();
-  // CHECK:STDERR:       |                  ^
+  // CHECK:STDERR: todo_fail_import_static.carbon:[[@LINE+2]]:18: note: used here [CppInteropParseNote]
   // CHECK:STDERR:
   Cpp.static_foo();
   //@dump-sem-ir-end