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/debug/DebugStringConvertible.cpp

@@ -10,8 +10,7 @@
 #include <folly/Conv.h>
 #include <folly/Format.h>
 
-namespace facebook {
-namespace react {
+namespace facebook::react {
 
 #if RN_DEBUG_STRING_CONVERTIBLE
 
@@ -150,5 +149,4 @@ std::string toString(void const *value) {
 
 #endif
 
-} // namespace react
-} // namespace facebook
+} // namespace facebook::react