Add frontend option -verbose-asm to emit verbose assembly.

This enables the option by default to aid readability with
-emit-assembly.

lib/Frontend/CompilerInvocation.cpp

@@ -3497,6 +3497,9 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
 
   Opts.FunctionSections = Args.hasArg(OPT_function_sections);
 
+  Opts.VerboseAsm = Args.hasFlag(OPT_verbose_asm, OPT_no_verbose_asm,
+                                 /*default*/ true);
+
   if (Args.hasArg(OPT_autolink_force_load))
     Opts.ForceLoadSymbolName = Args.getLastArgValue(OPT_module_link_name).str();