Emit mangled names for public symbols into the .swiftinterface

When the frontend option `-abi-comments-in-module-interface` is provided
during interface printing, the printed interface will contain
additional comments that provide the mangled names for public symbols.
This is an experiment in seeing how much information we can
meaningfully extract from a printed Swift interface for the purpose of
bridging with other languages.

lib/Frontend/CompilerInvocation.cpp

@@ -525,6 +525,7 @@ static void ParseModuleInterfaceArgs(ModuleInterfaceOptions &Opts,
     Args.hasArg(OPT_debug_emit_invalid_swiftinterface_syntax);
   Opts.PrintMissingImports =
     !Args.hasArg(OPT_disable_print_missing_imports_in_module_interface);
+  Opts.ABIComments = Args.hasArg(OPT_abi_comments_in_module_interface);
 
   if (const Arg *A = Args.getLastArg(OPT_library_level)) {
     StringRef contents = A->getValue();