tint: Add builtin tests for arguments passed by var

Change-Id: I81b69d23e40675d7f525e6369afec9aa0659d043
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/92321
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>

tools/src/cmd/intrinsic-gen/sem/sem.go

@@ -211,12 +211,16 @@ func (o Overload) Format(w fmt.State, verb rune) {
 
 // Parameter describes a single parameter of a function overload
 type Parameter struct {
-	Name string
-	Type FullyQualifiedName
+	Name    string
+	Type    FullyQualifiedName
+	IsConst bool // Did this parameter have a @const attribute?
 }
 
 // Format implements the fmt.Formatter interface
 func (p Parameter) Format(w fmt.State, verb rune) {
+	if p.IsConst {
+		fmt.Fprint(w, "@const ")
+	}
 	if p.Name != "" {
 		fmt.Fprintf(w, "%v: ", p.Name)
 	}