[Frontend] Add support for implicit import of _Backtracing

Once the API has gone through Swift Evolution, we will want to implicitly
import the _Backtracing module.  Add code to do that, but set it to off
by default for now.

rdar://105394140

lib/Frontend/CompilerInvocation.cpp

@@ -487,6 +487,11 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DisableImplicitStringProcessingModuleImport |=
     Args.hasArg(OPT_disable_implicit_string_processing_module_import);
 
+  Opts.DisableImplicitBacktracingModuleImport =
+    Args.hasFlag(OPT_disable_implicit_backtracing_module_import,
+                 OPT_enable_implicit_backtracing_module_import,
+                 true);
+
   if (Args.hasArg(OPT_enable_experimental_async_top_level))
     Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
                    "-enable-experimental-async-top-level");