tint: Deprecated module-scope 'let' for 'const'

Enable the parsing of 'const'.
Warn on use of module-scope 'let', and automatically replace with 'const'.

Fixed: tint:1580
Change-Id: I214aabca80686dc6b60ae21a7a57fbfb4898ea83
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/93786
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Kokoro: Kokoro <noreply+kokoro@google.com>

test/tint/const/inferred/function.wgsl.expected.glsl

@@ -0,0 +1,13 @@
+#version 310 es
+precision mediump float;
+
+layout(location = 0) out vec4 value;
+vec4 tint_symbol() {
+  return vec4(0.0f);
+}
+
+void main() {
+  vec4 inner_result = tint_symbol();
+  value = inner_result;
+  return;
+}