[embedded] Make -mergeable-symbols the default for Embedded Swift

Author: kubamracek

include/swift/Option/FrontendOptions.td

@@ -783,6 +783,10 @@ def mergeable_symbols : Flag<["-"], "mergeable-symbols">,
   Flags<[FrontendOption, NoInteractiveOption, HelpHidden]>,
   HelpText<"Emit symbol definitions as mergeable (linkonce_odr)">;
 
+def no_mergeable_symbols : Flag<["-"], "no-mergeable-symbols">,
+  Flags<[FrontendOption, NoInteractiveOption, HelpHidden]>,
+  HelpText<"Disable symbol definitions as mergeable (linkonce_odr)">;
+
 def disable_preallocated_instantiation_caches : Flag<["-"], "disable-preallocated-instantiation-caches">,
   Flags<[FrontendOption, NoInteractiveOption, HelpHidden]>,
   HelpText<"Avoid preallocating metadata instantiation caches in globals">;

lib/Frontend/CompilerInvocation.cpp

@@ -3718,9 +3718,8 @@ static bool ParseIRGenArgs(IRGenOptions &Opts, ArgList &Args,
 
   Opts.InternalizeSymbols = FrontendOpts.Static;
 
-  if (Args.hasArg(OPT_mergeable_symbols)) {
-    Opts.MergeableSymbols = true;
-  }
+  Opts.MergeableSymbols =
+      Args.hasFlag(OPT_mergeable_symbols, OPT_no_mergeable_symbols, true);
 
   if (Args.hasArg(OPT_disable_preallocated_instantiation_caches)) {
     Opts.NoPreallocatedInstantiationCaches = true;