workaround gcc ice

Author: huixie90

libcxx/test/std/containers/container.adaptors/flat.map/flat.map.modifiers/try_emplace_transparent.pass.cpp

@@ -7,6 +7,8 @@
 //===----------------------------------------------------------------------===//
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
+// gcc 15 ICE on this test
+// UNSUPPORTED: gcc
 
 // <flat_map>