feat(driver): color diagnostics like gcc with -fdiagnostics-color

Add -fdiagnostics-color[=auto|always|never] and -fno-diagnostics-color.
The default auto mode colors only a terminal stderr (TERM set and not
dumb on Unix; a console that accepts VT sequences on Windows), so test
harnesses, the desktop bundle and other pipes stay plain.

Every diagnostic label now comes from cps::diagnosticPrefix, which emits
gcc's default GCC_COLORS bytes: bold locus, bold red error/fatal error,
bold magenta warning, bold cyan note. compilation terminated., runtime
messages of the compiled program and lld output are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/tools/driver/NativeCompiler.cc b/tools/driver/NativeCompiler.cc
index e09fe98..324785f 100644
--- a/tools/driver/NativeCompiler.cc
+++ b/tools/driver/NativeCompiler.cc
@@ -1,5 +1,6 @@
 #include "NativeCompiler.h"
 #include "cps/CodeGen.h"
+#include "cps/Diagnostics.h"
 #include "clang/Basic/Diagnostic.h"
 #include "clang/Basic/DiagnosticOptions.h"
 #include "clang/Driver/Compilation.h"
@@ -48,7 +49,7 @@ struct TemporaryDirectory {
 };
 
 bool fail(const llvm::Twine &Message) {
-    llvm::errs() << "cpsc: fatal error: " << Message << "\n";
+    llvm::errs() << cps::diagnosticPrefix(cps::Severity::Fatal) << Message << "\n";
     return false;
 }
 
@@ -58,8 +59,10 @@ class DriverDiagnostics : public clang::DiagnosticConsumer {
         clang::DiagnosticConsumer::HandleDiagnostic(Level, Info);
         llvm::SmallString<256> Message;
         Info.FormatDiagnostic(Message);
-        llvm::errs() << "cpsc: " << (Level >= clang::DiagnosticsEngine::Error
-                                        ? "error: " : "warning: ") << Message << "\n";
+        llvm::errs() << cps::diagnosticPrefix(Level >= clang::DiagnosticsEngine::Error
+                                                  ? cps::Severity::Error
+                                                  : cps::Severity::Warning)
+                     << Message << "\n";
     }
 };