[gn] port 22629bb

Author: nico

llvm/utils/gn/secondary/libcxx/src/BUILD.gn

@@ -125,6 +125,7 @@ cxx_sources = [
   "condition_variable_destructor.cpp",
   "error_category.cpp",
   "exception.cpp",
+  "expected.cpp",
   "fstream.cpp",
   "functional.cpp",
   "future.cpp",