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/driver.cpp

@@ -24,8 +24,9 @@ struct Options {
 
   auto Build(CommandLine::CommandBuilder& b) -> void;
 
-  bool verbose;
-  bool fuzzing;
+  bool verbose = false;
+  bool fuzzing = false;
+  bool include_diagnostic_kind = false;
 
   ClangSubcommand clang;
   CompileSubcommand compile;
@@ -73,6 +74,16 @@ auto Options::Build(CommandLine::CommandBuilder& b) -> void {
       },
       [&](CommandLine::FlagBuilder& arg_b) { arg_b.Set(&fuzzing); });
 
+  b.AddFlag(
+      {
+          .name = "include-diagnostic-kind",
+          .help = R"""(
+When printing diagnostics, include the diagnostic kind as part of output. This
+applies to each message that forms a diagnostic, not just the primary message.
+)""",
+      },
+      [&](auto& arg_b) { arg_b.Set(&include_diagnostic_kind); });
+
   clang.AddTo(b, &selected_subcommand);
   compile.AddTo(b, &selected_subcommand);
   format.AddTo(b, &selected_subcommand);
@@ -84,8 +95,9 @@ auto Options::Build(CommandLine::CommandBuilder& b) -> void {
 
 auto Driver::RunCommand(llvm::ArrayRef<llvm::StringRef> args) -> DriverResult {
   if (driver_env_.installation->error()) {
-    *driver_env_.error_stream << "error: " << *driver_env_.installation->error()
-                              << "\n";
+    CARBON_DIAGNOSTIC(DriverInstallInvalid, Error, "{0}", std::string);
+    driver_env_.emitter.Emit(DriverInstallInvalid,
+                             driver_env_.installation->error()->str());
     return {.success = false};
   }
 
@@ -95,8 +107,15 @@ auto Driver::RunCommand(llvm::ArrayRef<llvm::StringRef> args) -> DriverResult {
       args, *driver_env_.output_stream, Options::Info,
       [&](CommandLine::CommandBuilder& b) { options.Build(b); });
 
+  // Regardless of whether the parse succeeded, try to use the diagnostic kind
+  // flag.
+  driver_env_.consumer.set_include_diagnostic_kind(
+      options.include_diagnostic_kind);
+
   if (!result.ok()) {
-    *driver_env_.error_stream << "error: " << result.error() << "\n";
+    CARBON_DIAGNOSTIC(DriverCommandLineParseFailed, Error, "{0}", std::string);
+    driver_env_.emitter.Emit(DriverCommandLineParseFailed,
+                             PrintToString(result.error()));
     return {.success = false};
   } else if (*result == CommandLine::ParseResult::MetaSuccess) {
     return {.success = true};