Add const-eval for `saturate`.

This CL adds const-eval for the `saturate` builtin.

Bug: tint:1581
Change-Id: I3729ea5b381b04b73bbe1bc8e03e5ce65c27e082
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/107362
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
diff --git a/test/tint/builtins/gen/var/saturate/a5b571.wgsl.expected.msl b/test/tint/builtins/gen/var/saturate/a5b571.wgsl.expected.msl
index dc1a973..24f08fd 100644
--- a/test/tint/builtins/gen/var/saturate/a5b571.wgsl.expected.msl
+++ b/test/tint/builtins/gen/var/saturate/a5b571.wgsl.expected.msl
@@ -2,7 +2,7 @@
 
 using namespace metal;
 void saturate_a5b571() {
-  float4 arg_0 = float4(1.0f);
+  float4 arg_0 = float4(2.0f);
   float4 res = saturate(arg_0);
 }