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/bug/fxc/gradient_in_varying_loop/1112.wgsl.expected.msl

@@ -4,6 +4,7 @@ using namespace metal;
 struct tint_symbol_2 {
   float2 vUV [[user(locn0)]];
 };
+
 struct tint_symbol_3 {
   float4 value [[color(0)]];
 };