Switch custom error stream output to diagnostic (#4846)

This switches most error printing to use diagnostics instead of direct
stream writes, even when not a specific file diagnostic. I'm allowing
empty filenames for this use-case.

This allows a little more specific testing to validate coverage of
output using the diagnostic coverage test. I'm adding a few tests to
cover things that weren't previously tested.

Separately, this also forces a little more standardization in format...
considering how changes like #4568 show effort being spent to _mirror_
diagnostic style, my thought is now to just use diagnostic code where
possible.

Note this also allows incrementally better testing of the language
server; I'm changing the crash fix from #4847 in favor of diagnostic
testing.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/driver/format_subcommand.cpp

@@ -56,9 +56,10 @@ auto FormatSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
   DriverResult result = {.success = true};
   if (options_.input_filenames.size() > 1 &&
       !options_.output_filename.empty()) {
-    *driver_env.error_stream
-        << "error: cannot format multiple input files when "
-           "--output is set\n";
+    CARBON_DIAGNOSTIC(FormatMultipleFilesToOneOutput, Error,
+                      "multiple input files are being provided; --output only "
+                      "works with one input");
+    driver_env.emitter.Emit(FormatMultipleFilesToOneOutput);
     result.success = false;
     return result;
   }
@@ -68,22 +69,20 @@ auto FormatSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
     result.per_file_success.back().second = false;
   };
 
-  StreamDiagnosticConsumer consumer(*driver_env.error_stream,
-                                    /*include_diagnostic_kind=*/false);
   for (auto& f : options_.input_filenames) {
     // Push a result, which we'll update on failure.
     result.per_file_success.push_back({f.str(), true});
 
     // TODO: Consider refactoring this for sharing with compile.
     // TODO: Decide what to do with `-` when there are multiple arguments.
-    auto source =
-        SourceBuffer::MakeFromFileOrStdin(*driver_env.fs, f, consumer);
+    auto source = SourceBuffer::MakeFromFileOrStdin(*driver_env.fs, f,
+                                                    driver_env.consumer);
     if (!source) {
       mark_per_file_error();
       continue;
     }
     SharedValueStores value_stores;
-    auto tokens = Lex::Lex(value_stores, *source, consumer);
+    auto tokens = Lex::Lex(value_stores, *source, driver_env.consumer);
 
     RawStringOstream buffer;
     if (Format::Format(tokens, buffer)) {