Use initializer list for composite initializations if backend.use_initializer_list is on.

spirv_glsl.cpp

@@ -5485,6 +5485,7 @@ void CompilerGLSL::emit_instruction(const Instruction &instruction)
 		{
 			// Only use this path if we are building composites.
 			// This path cannot be used for arithmetic.
+			constructor_op += type_to_glsl_constructor(get<SPIRType>(result_type));
 			constructor_op += "{ ";
 			if (splat)
 				constructor_op += to_expression(elems[0]);

spirv_msl.cpp

@@ -68,6 +68,7 @@ string CompilerMSL::compile()
 	backend.discard_literal = "discard_fragment()";
 	backend.swizzle_is_function = false;
 	backend.shared_is_implied = false;
+	backend.use_initializer_list = true;
 	backend.native_row_major_matrix = false;
 	backend.flexible_member_array_supported = false;
 	backend.force_temp_use_for_two_vector_shuffles = true;