[1.9>master] [MERGE #4952 @kfarnung] Set U_USING_ICU_NAMESPACE to 0 by default

Merge pull request #4952 from kfarnung:icu61

ICU61 updated the value of `U_USING_ICU_NAMESPACE` to 0 by default. In
order to maintain compatibility this change updates our ICU includes to
match and adds the `icu::` namespace explicitly when needed.

Refs: nodejs/node#18667
Refs: https://ssl.icu-project.org/trac/ticket/13460

Author: kfarnung

lib/Runtime/PlatformAgnostic/RuntimePlatformAgnosticPch.h

@@ -34,6 +34,7 @@
 #define FALSE 0
 #endif
 #else
+#define U_USING_ICU_NAMESPACE 0
 #include <unicode/umachine.h>
 #endif