Drop "diagnostic" from some filenames in the "diagnostics" folder (#6686)

Mainly because "sorting_diagnostic_consumer" is legacy, since
`SortingDiagnosticConsumer` became `SortingConsumer`. Also better
reflecting contents of these files.

Where I'm not renaming, I'm less positive about dropping "diagnostics"
from "file_diagnostics" and "null_diagnostics" (which contain both a
consumer and emitter, and "null.h" seems like poor naming), so not doing
that here. Also "diagnostic.h" contains `struct Diagnostic`, so is a
decent fit.

Assisted-by: Google Antigravity with Gemini 3 Flash

toolchain/diagnostics/consumer.cpp

@@ -0,0 +1,53 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/diagnostics/consumer.h"
+
+#include <algorithm>
+#include <cstdint>
+
+namespace Carbon::Diagnostics {
+
+auto StreamConsumer::HandleDiagnostic(Diagnostic diagnostic) -> void {
+  if (printed_diagnostic_) {
+    *stream_ << "\n";
+  } else {
+    printed_diagnostic_ = true;
+  }
+
+  for (const auto& message : diagnostic.messages) {
+    message.loc.FormatLocation(*stream_);
+    switch (message.level) {
+      case Level::Error:
+        *stream_ << "error: ";
+        break;
+      case Level::Warning:
+        *stream_ << "warning: ";
+        break;
+      case Level::Note:
+        *stream_ << "note: ";
+        break;
+      case Level::LocationInfo:
+        break;
+    }
+    *stream_ << message.Format();
+    if (include_diagnostic_kind_) {
+      *stream_ << " [" << message.kind << "]";
+    }
+    *stream_ << "\n";
+    // Don't include a snippet for location information to keep this diagnostic
+    // more visually associated with the following diagnostic that it describes
+    // and to better match C++ compilers.
+    if (message.level != Level::LocationInfo) {
+      message.loc.FormatSnippet(*stream_);
+    }
+  }
+}
+
+auto ConsoleConsumer() -> Consumer& {
+  static auto* consumer = new StreamConsumer(&llvm::errs());
+  return *consumer;
+}
+
+}  // namespace Carbon::Diagnostics