SILGen: Lower keypath references to other modules' properties and subscripts as external keypath components.

This way we'll link against the key path component the other module provides instead of making fragile assumptions about its current implementation. Since external keypath lowering isn't fully implemented elsewhere in the compiler, this is enabled behind a staging flag.

external keypath staging

lib/Frontend/CompilerInvocation.cpp

@@ -326,6 +326,8 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                    Target.isOSDarwin());
   Opts.EnableSILOpaqueValues |= Args.hasArg(OPT_enable_sil_opaque_values);
 
+  Opts.EnableKeyPathResilience |= Args.hasArg(OPT_enable_key_path_resilience);
+  
 #if SWIFT_DARWIN_ENABLE_STABLE_ABI_BIT
   Opts.UseDarwinPreStableABIBit = false;
 #else