Implement the extension GL_AMD_gpu_shader_half_float_fetch

- Support new opaque types: f16sampler*, f16image*, f16subpassInput*.
- Add new built-in GLSL texture/image functions.

glslang/Include/Types.h

@@ -205,6 +205,9 @@ struct TSampler {   // misnomer now; includes images, textures without sampler,
 
         switch (type) {
         case EbtFloat:               break;
+#ifdef AMD_EXTENSIONS
+        case EbtFloat16: s.append("f16"); break;
+#endif
         case EbtInt:  s.append("i"); break;
         case EbtUint: s.append("u"); break;
         default:  break;  // some compilers want this