writer/hlsl: Don't explode on unhandled member decorations

[[align(n)]], [[size(n)]] are valid decorations that should not cause the writer to fail.
Instead of allow-listing these, just handle the cases it actually cares about.

Fixed: tint:686
Change-Id: I7c60d251fcaee424fe1c1a1f1f58123eea8c99aa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/46380
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/writer/hlsl/generator_impl.cc

diff --git a/src/writer/hlsl/generator_impl.cc b/src/writer/hlsl/generator_impl.cc
index f969e5f..f7a7e96 100644
--- a/src/writer/hlsl/generator_impl.cc
+++ b/src/writer/hlsl/generator_impl.cc
@@ -2610,8 +2610,7 @@
       out << " " << namer_.NameFor(builder_.Symbols().NameFor(mem->symbol()));
     }
 
-    if (mem->decorations().size() > 0) {
-      auto* deco = mem->decorations()[0];
+    for (auto* deco : mem->decorations()) {
       if (auto* location = deco->As<ast::LocationDecoration>()) {
         out << " : TEXCOORD" << location->value();
       } else if (auto* builtin = deco->As<ast::BuiltinDecoration>()) {
@@ -2621,11 +2620,6 @@
           return false;
         }
         out << " : " << attr;
-      } else if (deco->Is<ast::StructMemberOffsetDecoration>()) {
-        // Nothing to do, offsets are handled at the point of access.
-      } else {
-        diagnostics_.add_error("unsupported struct member decoration");
-        return false;
       }
     }