[Backtracing] Don't add to the LTO library list when disabled.

When backtracing is disabled, don't add it to the LTO library list that
we use it `stdlib_lto` is set.

rdar://106459980

Author: al45tair

test/lit.cfg

@@ -1092,13 +1092,15 @@ if run_vendor == 'apple':
     # tests.
     # rdar://70787171
     if "stdlib_lto" in config.available_features:
-        for library in ["swiftCore", "swiftStdlibUnittest",
-                        "swiftStdlibUnicodeUnittest",
-                        "swiftStdlibCollectionUnittest",
-                        "swiftSwiftPrivateLibcExtras", "swiftSwiftPrivate",
-                        "swiftDarwin", "swiftSwiftPrivateThreadExtras",
-                        "swiftSwiftOnoneSupport", "swift_Concurrency",
-                        "swift_Backtracing"]:
+        libraries = ["swiftCore", "swiftStdlibUnittest",
+                     "swiftStdlibUnicodeUnittest",
+                     "swiftStdlibCollectionUnittest",
+                     "swiftSwiftPrivateLibcExtras", "swiftSwiftPrivate",
+                     "swiftDarwin", "swiftSwiftPrivateThreadExtras",
+                     "swiftSwiftOnoneSupport", "swift_Concurrency"]
+        if backtracing is not None:
+            libraries.append('swift_Backtracing')
+        for library in libraries:
             swift_execution_tests_extra_flags += ' -Xlinker -l%s'% library
 
         swift_native_clang_tools_path = lit_config.params.get('swift_native_clang_tools_path', None)