[libcxx] [libcxxabi] [libunwind] [llvm] [libc++] Upgrade to GCC 15 (PR #138293)

[Louis Dionne via llvm-commits]

================
@@ -168,3 +168,6 @@ void instantiate() {
   void_t<int>();
 #endif
 }
+
+// This is not a .compile.pass.cpp, because GCC only complains in CodeGen about incorrect builtins usage
+int main() {}
----------------
ldionne wrote:

```suggestion
int main(int, char**) { return 0; }
```

https://github.com/llvm/llvm-project/pull/138293