[Driver] -g implies -enable-anonymous-context-mangled-names.

Including the mangled names in anonymous context metadata can bloat
code size, and is not needed for normal runtime resolution. However,
it aids debugging, so have -g implies the emission of this extra
metadata, which is always optional.

Author: DougGregor

lib/Driver/ToolChains.cpp

@@ -244,6 +244,12 @@ static void addCommonFrontendArgs(const ToolChain &TC, const OutputInfo &OI,
     arguments.push_back(inputArgs.MakeArgString(workingDirectory));
   }
 
+  // -g implies -enable-anonymous-context-mangled-names, because the extra
+  // metadata aids debugging.
+  if (inputArgs.hasArg(options::OPT_g)) {
+    arguments.push_back("-enable-anonymous-context-mangled-names");
+  }
+
   // Pass through any subsystem flags.
   inputArgs.AddAllArgs(arguments, options::OPT_Xllvm);
   inputArgs.AddAllArgs(arguments, options::OPT_Xcc);

test/Driver/debug_anonymous_context_metadata.swift

@@ -0,0 +1,4 @@
+// RUN: %target-swiftc_driver -### -g %s 2>&1 | %FileCheck %s
+
+// CHECK: -enable-anonymous-context-mangled-names
+