[Validation] v-0006: variables must be defined before use

Bug:tint 6
Change-Id: I22f3117a8d59eaba97166de1f188156a9e3cd7a0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/26381
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Sarah Mashayekhi <sarahmashay@google.com>

test/undefined-variable-global-scope.wgsl

@@ -0,0 +1,23 @@
+# Copyright 2020 The Tint Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# Vertex shader
+var<private> a : i32;
+
+fn main() -> void {
+  a = 2;
+  return;
+}
+entry_point vertex as "main" = main;
+

test/undefined-variable-inner-block.wgsl

@@ -0,0 +1,20 @@
+# Copyright 2020 The Tint Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+fn main() -> void {
+    var a : f32 = 2.0;
+   { a = 3.14;}
+    return;
+}
+entry_point fragment = main;