squash! WIP: fix(msl-out): don't check member.binding in `write_typ…

…e_defs`, `should_pack_struct_member`

Add tests!

naga/tests/in/struct-layout.wgsl

@@ -0,0 +1,43 @@
+// Create several type definitions to test `align` and `size` layout.
+
+struct NoPadding {
+	@location(0)
+	v3: vec3f, // align 16, size 12; no start padding needed
+	@location(1)
+	f3: f32, // align 4, size 4; no start padding needed
+}
+// struct NoPaddingNoBindings {
+//	v3: vec3f,
+//	f3: f32,
+// }
+@fragment
+fn no_padding_frag(input: NoPadding) -> @location(0) vec4f {
+	_ = input;
+	return vec4f(0.0);
+}
+@vertex
+fn no_padding_vert(input: NoPadding) -> @builtin(position) vec4f {
+	_ = input;
+	return vec4f(0.0);
+}
+
+struct NeedsPadding {
+	@location(0) f3_forces_padding: f32, // align 4, size 4; no start padding needed
+	@location(1) v3_needs_padding: vec3f, // align 16, size 12; needs 12 bytes of padding
+	@location(2) f3: f32, // align 4, size 4; no start padding needed
+}
+// struct NeedsPaddingNoBindings {
+//	f3_forces_padding: f32,
+//	v3_needs_padding: vec3f,
+//	f3: f32,
+// }
+@fragment
+fn needs_padding_frag(input: NeedsPadding) -> @location(0) vec4f {
+	_ = input;
+	return vec4f(0.0);
+}
+@vertex
+fn needs_padding_vert(input: NeedsPadding) -> @builtin(position) vec4f {
+	_ = input;
+	return vec4f(0.0);
+}