Use nested namespace name

Summary:
changelog: [internal]

Enable two new clang tidy checks:
- modernize-concat-nested-namespaces
- google-build-using-namespace

jest_e2e[run_all_tests]

Reviewed By: rshest

Differential Revision: D40020646

fbshipit-source-id: f3be80b5f829dd0ba376bdd70ed13332e114d48a

ReactCommon/react/renderer/attributedstring/AttributedStringBox.cpp

@@ -11,8 +11,7 @@
 
 #include <utility>
 
-namespace facebook {
-namespace react {
+namespace facebook::react {
 
 AttributedStringBox::AttributedStringBox()
     : mode_(Mode::Value),
@@ -86,5 +85,4 @@ bool operator!=(
   return !(lhs == rhs);
 }
 
-} // namespace react
-} // namespace facebook
+} // namespace facebook::react