tint/resolver: Add missing ResolvedIdentifier case for 'let'

Prevents an ICE.
I had this trigger, but I'm now struggling to find a reproduction.

Only used to print a diagnostic when things go wrong.

Change-Id: Iab78c3bcaaf1afb2f556247f8a419f93a727ec96
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/122341
Kokoro: Ben Clayton <bclayton@chromium.org>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@chromium.org>
diff --git a/src/tint/resolver/dependency_graph.cc b/src/tint/resolver/dependency_graph.cc
index 9b6d471..8292adb 100644
--- a/src/tint/resolver/dependency_graph.cc
+++ b/src/tint/resolver/dependency_graph.cc
@@ -37,6 +37,7 @@
 #include "src/tint/ast/internal_attribute.h"
 #include "src/tint/ast/interpolate_attribute.h"
 #include "src/tint/ast/invariant_attribute.h"
+#include "src/tint/ast/let.h"
 #include "src/tint/ast/location_attribute.h"
 #include "src/tint/ast/loop_statement.h"
 #include "src/tint/ast/must_use_attribute.h"
@@ -810,6 +811,9 @@
             [&](const ast::Var* n) {  //
                 return "var '" + symbols.NameFor(n->name->symbol) + "'";
             },
+            [&](const ast::Let* n) {  //
+                return "let '" + symbols.NameFor(n->name->symbol) + "'";
+            },
             [&](const ast::Const* n) {  //
                 return "const '" + symbols.NameFor(n->name->symbol) + "'";
             },