[tint] Move cached builtin info out of Symbol

Use a hashmap local to the dependency analysis instead. This breaks
the dependency from `symbol` to `builtin`, allowing us to use Symbol
from the builtin namespace.

Change-Id: I4f5d3d59e56c86f0ab833a41d97966dab5fbd655
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/141721
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/docs/tint/layering.md b/docs/tint/layering.md
index 587febc..3e3c8f6 100644
--- a/docs/tint/layering.md
+++ b/docs/tint/layering.md
@@ -50,6 +50,11 @@
 |               Types                     |      |
 +-----------------------------------------+      |
                   |                              |
+                  V                              |
++-----------------------------------------+      |
+|              Builtin                    |      |
++-----------------------------------------+      |
+                  |                              |
                   |------------------------------+
                   V
 +-----------------------------------------+
@@ -57,11 +62,6 @@
 +-----------------------------------------+
                   |
                   V
-+-----------------------------------------+
-|              Builtin                    |
-+-----------------------------------------+
-                  |
-                  V
 +-----------------------------------------+ +-------------+
 |               Utils                     | | Initializer |
 +-----------------------------------------+ +-------------+