writers: Use the new sem::Module::DependencyOrderedDeclarations

As the resolver currently enforces in-order declarations, this does not
change the declaration order from iterating over the
ast::Module::GlobalDeclarations.

The MSL backend has been changed to use the
sem::Module::DependencyOrderedDeclarations list instead of looping over
different declaration types separately.

Bug: tint:1266
Change-Id: I698d612032285311017bfceab3c42adae1928a0e
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/79767
Reviewed-by: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/var/override/numbered/zero_init/bool.wgsl.expected.msl

@@ -2,6 +2,7 @@
 
 using namespace metal;
 constant bool o [[function_constant(1234)]];
+
 kernel void tint_symbol() {
   return;
 }

test/var/override/numbered/zero_init/f32.wgsl.expected.msl

@@ -2,6 +2,7 @@
 
 using namespace metal;
 constant float o [[function_constant(1234)]];
+
 kernel void tint_symbol() {
   return;
 }

test/var/override/numbered/zero_init/i32.wgsl.expected.msl

@@ -2,6 +2,7 @@
 
 using namespace metal;
 constant int o [[function_constant(1234)]];
+
 kernel void tint_symbol() {
   return;
 }

test/var/override/numbered/zero_init/u32.wgsl.expected.msl

@@ -2,6 +2,7 @@
 
 using namespace metal;
 constant uint o [[function_constant(1234)]];
+
 kernel void tint_symbol() {
   return;
 }