clang/win: Make diagnostics colored if ansicon is around at runhooks time.

build/config/compiler/BUILD.gn

Index: build/config/compiler/BUILD.gn
diff --git a/build/config/compiler/BUILD.gn b/build/config/compiler/BUILD.gn
index 466b2b95000a68f3050fbd9f4fc5e657137ed1de..74accae9493c1f841c57dc37333a4cd425c13285 100644
--- a/build/config/compiler/BUILD.gn
+++ b/build/config/compiler/BUILD.gn
@@ -130,6 +130,14 @@ config("compiler") {
       } else {
         cflags += [ "-m64" ]
       }
+      if (exec_script("//build/win/use_ansi_codes.py", [], "trim string") ==
+          "True") {
+        cflags += [
+          # cmd.exe doesn't understand ANSI escape codes by default,
+          # so only enable them if something emulating them is around.
+          "-fansi-escape-codes",
+        ]
+      }
     }
   } else {
     # Common GCC compiler flags setup.