[SPIR-V] Allow `using` and `using namespace` declarations (#6229)

Fixes #5938

tools/clang/lib/SPIRV/SpirvEmitter.cpp

@@ -1025,6 +1025,10 @@ void SpirvEmitter::doDecl(const Decl *decl) {
     doClassTemplateDecl(classTemplateDecl);
   } else if (isa<FunctionTemplateDecl>(decl)) {
     // nothing to do.
+  } else if (isa<UsingDecl>(decl)) {
+    // nothing to do.
+  } else if (isa<UsingDirectiveDecl>(decl)) {
+    // nothing to do.
   } else {
     emitError("decl type %0 unimplemented", decl->getLocation())
         << decl->getDeclKindName();

tools/clang/test/CodeGenSPIRV/using.hlsl

@@ -0,0 +1,23 @@
+// RUN: %dxc -T ps_6_0 -E main -fcgl  %s -spirv | FileCheck %s
+
+namespace n {
+    using f = float;
+    namespace n2 {
+       f foo(f a) { return sin(a); }
+    }
+
+}
+
+using namespace n;
+using f32 = n::f;
+
+
+using n2::foo;
+
+// CHECK: %src_main = OpFunction %float
+// CHECK-NEXT: %a = OpFunctionParameter %_ptr_Function_float
+f32 main(f32 a:A) : SV_Target {
+
+  // CHECK: OpFunctionCall %float %n__n2__foo %param_var_a_0
+  return foo(a);
+}