Work around bug in old clang versions that had __has_cpp_attribute defined even
in C mode.

https://discourse.llvm.org/t/possible-bug-with-has-cpp-attribute/36508/4
https://bugs.llvm.org/show_bug.cgi?id=23435
--- common/unicode/platform.h.orig	2026-03-17 12:44:50.000000000 -0500
+++ common/unicode/platform.h	2026-03-22 14:39:11.000000000 -0500
@@ -388,7 +388,7 @@
 #else
 #   define UPRV_HAS_ATTRIBUTE(x) 0
 #endif
-#ifdef __has_cpp_attribute
+#if defined(__cplusplus) && defined(__has_cpp_attribute)
 #   define UPRV_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
 #else
 #   define UPRV_HAS_CPP_ATTRIBUTE(x) 0