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/StandardLibrary.cc b/tools/driver/StandardLibrary.cc
index ac88745..7680054 100644
--- a/tools/driver/StandardLibrary.cc
+++ b/tools/driver/StandardLibrary.cc
@@ -1,4 +1,5 @@
 #include "StandardLibrary.h"
+#include "cps/Diagnostics.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
@@ -19,7 +20,8 @@ StandardLibrary::~StandardLibrary() {
 bool StandardLibrary::load(std::vector<std::string> &Inputs) {
     auto Error = llvm::sys::fs::createUniqueDirectory("cpsc-std", Directory);
     if (Error) {
-        llvm::errs() << "cpsc: fatal error: cannot extract standard library: " << Error.message() << "\n";
+        llvm::errs() << cps::diagnosticPrefix(cps::Severity::Fatal)
+                     << "cannot extract standard library: " << Error.message() << "\n";
         return false;
     }
     for (const auto &File : LibraryFiles) {
@@ -32,8 +34,9 @@ bool StandardLibrary::load(std::vector<std::string> &Inputs) {
             Error = Output.error();
         }
         if (Error) {
-            llvm::errs() << "cpsc: fatal error: cannot extract standard library file '"
-                         << File.Name << "': " << Error.message() << "\n";
+            llvm::errs() << cps::diagnosticPrefix(cps::Severity::Fatal)
+                         << "cannot extract standard library file '" << File.Name
+                         << "': " << Error.message() << "\n";
             Output.clear_error();
             return false;
         }