[type-determiner] return false when type determining an undeclared function

Change-Id: Ia7e43be64675528037f92026a6c239d1e5220fc0
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/26941
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>

test/function-must-be-declared-before-use-fail.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.
+
+fn main() -> void {
+    var a:f32 = func1();
+    return;
+}
+
+fn func1() -> f32 {
+    var a:i32 = 2.1;
+    return a;
+}