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/expressions/binary/mul/mat3x2-vec3/f32.wgsl.expected.glsl

@@ -1,6 +1,10 @@
 #version 310 es
 precision mediump float;
 
+struct S {
+  mat3x2 matrix;
+  vec3 vector;
+};
 
 layout (binding = 0) uniform S_1 {
   mat3x2 matrix;

test/expressions/binary/mul/mat3x3-vec3/f32.wgsl.expected.glsl

@@ -1,6 +1,10 @@
 #version 310 es
 precision mediump float;
 
+struct S {
+  mat3 matrix;
+  vec3 vector;
+};
 
 layout (binding = 0) uniform S_1 {
   mat3 matrix;

test/expressions/binary/mul/vec3-mat3x3/f32.wgsl.expected.glsl

@@ -1,6 +1,10 @@
 #version 310 es
 precision mediump float;
 
+struct S {
+  mat3 matrix;
+  vec3 vector;
+};
 
 layout (binding = 0) uniform S_1 {
   mat3 matrix;

test/expressions/binary/mul/vec3-mat4x3/f32.wgsl.expected.glsl

@@ -1,6 +1,10 @@
 #version 310 es
 precision mediump float;
 
+struct S {
+  mat4x3 matrix;
+  vec3 vector;
+};
 
 layout (binding = 0) uniform S_1 {
   mat4x3 matrix;