Enhance GN's "desc" command to support configs.

tools/gn/switches.h

Index: tools/gn/switches.h
diff --git a/tools/gn/switches.h b/tools/gn/switches.h
index cebb19c98e1a2205abab598a396c792c969fa0fd..b86bf330569cf8b183503d32f70a366363dd4746 100644
--- a/tools/gn/switches.h
+++ b/tools/gn/switches.h
@@ -84,6 +84,20 @@ extern const char kVersion[];
 extern const char kVersion_HelpShort[];
 extern const char kVersion_Help[];
 
+// This switch is used by several commands. It is here so it can be shared,
+// but it's documented in the individual commands it applies to rather than
+// globally.
+extern const char kAllToolchains[];
+#define ALL_TOOLCHAINS_SWITCH_HELP \
+  "  --all-toolchains\n" \
+  "      Normally only inputs in the default toolchain will be included.\n" \
+  "      This switch will turn on matching all toolchains.\n" \
+  "\n" \
+  "      For example, a file is in a target might be compiled twice:\n" \
+  "      once in the default toolchain and once in a secondary one. Without\n" \
+  "      this flag, only the default toolchain one will be matched by\n" \
+  "      wildcards. With this flag, both will be matched.\n"
+
 }  // namespace switches
 
 #endif  // TOOLS_GN_SWITCHES_H_