Move sem::BuiltinType to builtin::Function.

This CL moves `sem::BuiltinType` to `builtin::Function`. This makes it
available for use in the IR. The `Function` name better maps to the
usage in that it contains the builtin functions.

Bug: tint:1834
Change-Id: Ic1a26525d7845d79eb6239bdeb2b73e05f586c24
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/122606
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
diff --git a/src/tint/resolver/dependency_graph.cc b/src/tint/resolver/dependency_graph.cc
index f718a46..67eb430 100644
--- a/src/tint/resolver/dependency_graph.cc
+++ b/src/tint/resolver/dependency_graph.cc
@@ -440,7 +440,8 @@
         auto* resolved = scope_stack_.Get(to);
         if (!resolved) {
             auto s = symbols_.NameFor(to);
-            if (auto builtin_fn = sem::ParseBuiltinType(s); builtin_fn != sem::BuiltinType::kNone) {
+            if (auto builtin_fn = builtin::ParseFunction(s);
+                builtin_fn != builtin::Function::kNone) {
                 graph_.resolved_identifiers.Add(from, ResolvedIdentifier(builtin_fn));
                 return;
             }
@@ -838,7 +839,7 @@
                 return "<unknown>";
             });
     }
-    if (auto builtin_fn = BuiltinFunction(); builtin_fn != sem::BuiltinType::kNone) {
+    if (auto builtin_fn = BuiltinFunction(); builtin_fn != builtin::Function::kNone) {
         return "builtin function '" + utils::ToString(builtin_fn) + "'";
     }
     if (auto builtin_ty = BuiltinType(); builtin_ty != builtin::Builtin::kUndefined) {