tint: Implement const-eval of modf

Bug: tint:1581
Change-Id: I53151ebf43601cd6afcdd2ec91d0ff9c4e650ef3
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/111241
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/test/tint/builtins/gen/var/modf/bbf7f7.wgsl.expected.msl b/test/tint/builtins/gen/var/modf/bbf7f7.wgsl.expected.msl
index 2ef6251..3e4accd 100644
--- a/test/tint/builtins/gen/var/modf/bbf7f7.wgsl.expected.msl
+++ b/test/tint/builtins/gen/var/modf/bbf7f7.wgsl.expected.msl
@@ -13,7 +13,7 @@
 }
 
 void modf_bbf7f7() {
-  float arg_0 = 1.0f;
+  float arg_0 = -1.5f;
   modf_result res = tint_modf(arg_0);
 }