Fix compiler errors from clang versions older than 15 not supporting gnu style attributes on namespaces

Fixes errors in GVN.h after changes in llvm#96630 to how LLVM_LIBRARY_VISIBILITY is defined

Author: fsfod

llvm/include/llvm/Support/Compiler.h

@@ -130,7 +130,12 @@
 
 #if (!(defined(_WIN32) || defined(__CYGWIN__)) ||                              \
      (defined(__MINGW32__) && defined(__clang__)))
+// Clang compilers older then 15 do not support gnu style attributes on namespaces
+#if defined(__clang__) && _clang_major__ < 15
+#define LLVM_LIBRARY_VISIBILITY [[gnu::visibility("hidden")]]
+#else
 #define LLVM_LIBRARY_VISIBILITY LLVM_ATTRIBUTE_VISIBILITY_HIDDEN
+#endif
 #define LLVM_ALWAYS_EXPORT LLVM_ATTRIBUTE_VISIBILITY_DEFAULT
 #elif defined(_WIN32)
 #define LLVM_ALWAYS_EXPORT __declspec(dllexport)