glsl: Always emit structures

Skipping those that are block-decorated is not correct, as
block-decorated structures can also have non-buffer usages. This is
even clearer now that WGSL has removed the block attribute.

Bug: tint:1324
Change-Id: I6484766a5c541d39e2dc08beb3ae7b889759a3fb
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/72083
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

test/statements/assign/phony/storage_buffer.wgsl.expected.glsl

@@ -1,6 +1,9 @@
 #version 310 es
 precision mediump float;
 
+struct S {
+  int i;
+};
 
 layout (binding = 0) buffer S_1 {
   int i;

test/statements/assign/phony/uniform_buffer.wgsl.expected.glsl

@@ -1,6 +1,9 @@
 #version 310 es
 precision mediump float;
 
+struct S {
+  int i;
+};
 
 layout (binding = 0) uniform S_1 {
   int i;