ICU-20209 Fix build failures on Windows with std::atomic not in enclosing namespace std

Also: Change to use the ICU namespace macro instead, and skip doxygen as well.

icu4c/source/common/umutex.h

@@ -54,15 +54,23 @@ U_NAMESPACE_END
 
 #include <atomic>
 
-U_NAMESPACE_BEGIN
-
 // Export an explicit template instantiation of std::atomic<int32_t>. 
 // When building DLLs for Windows this is required as it is used as a data member of the exported SharedObject class.
 // See digitlst.h, pluralaffix.h, datefmt.h, and others for similar examples.
-#if U_PF_WINDOWS <= U_PLATFORM && U_PLATFORM <= U_PF_CYGWIN
+#if U_PF_WINDOWS <= U_PLATFORM && U_PLATFORM <= U_PF_CYGWIN && !defined(U_IN_DOXYGEN)
+  #if defined(__clang__)
+  // Suppress the warning that the explicit instantiation after explicit specialization has no effect.
+  #pragma clang diagnostic push
+  #pragma clang diagnostic ignored "-Winstantiation-after-specialization"
+  #endif
 template struct U_COMMON_API std::atomic<int32_t>;
+  #if defined(__clang__)
+  #pragma clang diagnostic pop
+  #endif
 #endif
 
+U_NAMESPACE_BEGIN
+
 typedef std::atomic<int32_t> u_atomic_int32_t;
 #define ATOMIC_INT32_T_INITIALIZER(val) ATOMIC_VAR_INIT(val)
 

icu4c/source/i18n/unicode/numberrangeformatter.h

@@ -185,8 +185,14 @@ class NumberRangeFormatterImpl;
  * Export an explicit template instantiation. See datefmt.h
  * (When building DLLs for Windows this is required.)
  */
-#if U_PF_WINDOWS <= U_PLATFORM && U_PLATFORM < U_PF_CYGWIN && !defined(U_IN_DOXYGEN)
-template struct U_I18N_API std::atomic<impl::NumberRangeFormatterImpl*>;
+#if U_PLATFORM == U_PF_WINDOWS && !defined(U_IN_DOXYGEN)
+} // namespace icu::number
+U_NAMESPACE_END
+
+template struct U_I18N_API std::atomic< U_NAMESPACE_QUALIFIER number::impl::NumberRangeFormatterImpl*>;
+
+U_NAMESPACE_BEGIN
+namespace number {  // icu::number
 #endif
 /** \endcond */
 

icu4c/source/test/intltest/numbertest.h

@@ -10,6 +10,7 @@
 #include "intltest.h"
 #include "number_affixutils.h"
 #include "numparse_stringsegment.h"
+#include "numrange_impl.h"
 #include "unicode/locid.h"
 #include "unicode/numberformatter.h"
 #include "unicode/numberrangeformatter.h"