Update ICU C++ API symbols

Recent ICU versions have added U_SHOW_CPLUSPLUS_HEADER_API, and we need
to set this to zero as well to hide the ICU C++ APIs from pg_locale.h

Per discussion, we want cpluspluscheck to work cleanly in backbranches,
so backpatch both this and its predecessor commit ed26c4e25a to all
supported versions.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/1115793.1754414782%40sss.pgh.pa.us
Backpatch-through: 13

.cirrus.tasks.yml

@@ -773,14 +773,11 @@ task:
   # - Don't use ccache, the files are uncacheable, polluting ccache's
   #   cache
   # - Use -fmax-errors, as particularly cpluspluscheck can be very verbose
-  # - XXX have to disable ICU to avoid errors:
-  #   https://postgr.es/m/20220323002024.f2g6tivduzrktgfa%40alap3.anarazel.de
   ###
   always:
     headers_headerscheck_script: |
       time ./configure \
         ${LINUX_CONFIGURE_FEATURES} \
-        --without-icu \
         --quiet \
         CC="gcc" CXX"=g++" CLANG="clang"
       make -s -j${BUILD_JOBS} clean

src/include/utils/pg_locale.h

@@ -16,6 +16,11 @@
 #include <xlocale.h>
 #endif
 #ifdef USE_ICU
+/* only include the C APIs, to avoid errors in cpluspluscheck */
+#undef U_SHOW_CPLUSPLUS_API
+#define U_SHOW_CPLUSPLUS_API 0
+#undef U_SHOW_CPLUSPLUS_HEADER_API
+#define U_SHOW_CPLUSPLUS_HEADER_API 0
 #include <unicode/ucol.h>
 #endif