[libcxx] [test] XFAIL the get/put long_double_ru_RU tests on Glibc < 2.27

Those older versions used a different monetary decimal separator.
To avoid unnecessary churn to support that, just XFAIL the test
on those older versions. (Up until
df1e43c496, the whole test was XFAILed
on all versions of glibc.)

Differential Revision: https://reviews.llvm.org/D120979

libcxx/test/std/localization/locale.categories/category.monetary/locale.money.get/locale.money.get.members/get_long_double_ru_RU.pass.cpp

@@ -11,6 +11,8 @@
 
 // REQUIRES: locale.ru_RU.UTF-8
 
+// XFAIL: glibc-old-ru_RU-decimal-point
+
 // <locale>
 
 // class money_get<charT, InputIterator>

libcxx/test/std/localization/locale.categories/category.monetary/locale.money.put/locale.money.put.members/put_long_double_ru_RU.pass.cpp

@@ -11,6 +11,8 @@
 
 // REQUIRES: locale.ru_RU.UTF-8
 
+// XFAIL: glibc-old-ru_RU-decimal-point
+
 // <locale>
 
 // class money_put<charT, OutputIterator>

libcxx/utils/libcxx/test/features.py

@@ -105,6 +105,18 @@ DEFAULT_FEATURES = [
             }
           """)),
 
+  # Check for Glibc < 2.27, where the ru_RU.UTF-8 locale had
+  # mon_decimal_point == ".", which our tests don't handle.
+  Feature(name='glibc-old-ru_RU-decimal-point',
+          when=lambda cfg: not '_LIBCPP_HAS_NO_LOCALIZATION' in compilerMacros(cfg) and not programSucceeds(cfg, """
+            #include <locale.h>
+            #include <string.h>
+            int main(int, char**) {
+              setlocale(LC_ALL, "ru_RU.UTF-8");
+              return strcmp(localeconv()->mon_decimal_point, ",");
+            }
+          """)),
+
   # Whether Bash can run on the executor.
   # This is not always the case, for example when running on embedded systems.
   #