GLSL: implement host-visible memory padding.

Since GLSL ES does not support the offset= attribute, struct members
with explicit @align or @size attributes require adding explicit
padding members.  This in turn requires rewriting any constructor
calls to initialize the new padding to zero, handled in the same
transform.

Note that this is currently overly-verbose, and will add padding where
GLSL doesn't technically need it (e.g., padding a vec3 out to 16 bytes).

Bug: tint:1415
Change-Id: Ia9ba513066a0e84f4c43247fcbbe02f5fadd6630
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/101720
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/buffer/storage/static_index/read.wgsl.expected.glsl

@@ -15,6 +15,7 @@ layout(binding = 0, std430) buffer S_ssbo {
   mat3x2 h;
   Inner i;
   Inner j[4];
+  uint pad;
 } s;
 
 void tint_symbol() {

test/tint/buffer/storage/static_index/write.wgsl.expected.glsl

@@ -15,6 +15,7 @@ layout(binding = 0, std430) buffer S_ssbo {
   mat3x2 h;
   Inner i;
   Inner j[4];
+  uint pad;
 } s;
 
 void tint_symbol() {