Backends: Fixed various warnings discovered when using MinGW GCC 15/Clang on latest backends.

dx12: 'ImGui_ImplDX12_Data* bd' shadowed local in spite of being in lambda.

backends/imgui_impl_dx11.cpp

@@ -52,6 +52,12 @@
 #pragma comment(lib, "d3dcompiler") // Automatically link with d3dcompiler.lib as we are using D3DCompile() below.
 #endif
 
+// Clang/GCC warnings with -Weverything
+#if defined(__clang__)
+#pragma clang diagnostic ignored "-Wold-style-cast"         // warning: use of old-style cast                            // yes, they are more terse.
+#pragma clang diagnostic ignored "-Wsign-conversion"        // warning: implicit conversion changes signedness
+#endif
+
 // DirectX11 data
 struct ImGui_ImplDX11_Texture
 {