validation: Reject decorations on local variables

These are never valid. The WGSL parser cannot produce them, but the
SPIR-V reader can since these are not always caught by spirv-val.

Fixed: chromium:1239557
Change-Id: Ie19e4534ffb73b61beaa42046b18b2b8a3f7f65b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/62020
Auto-Submit: James Price <jrprice@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Sarah Mashayekhi <sarahmashay@google.com>
Reviewed-by: Sarah Mashayekhi <sarahmashay@google.com>

src/resolver/resolver.cc

diff --git a/src/resolver/resolver.cc b/src/resolver/resolver.cc
index be8e45a..3eb9629 100644
--- a/src/resolver/resolver.cc
+++ b/src/resolver/resolver.cc
@@ -3366,8 +3366,11 @@
   }
 
   for (auto* deco : var->decorations()) {
-    // TODO(bclayton): Validate decorations
     Mark(deco);
+    if (!deco->Is<ast::InternalDecoration>()) {
+      AddError("decorations are not valid on local variables", deco->source());
+      return false;
+    }
   }
 
   variable_stack_.set(var->symbol(), info);