[spirv-writer] Call the Call expression generator.

This CL adds the missing call for the call expression.

Bug: tint:5
Change-Id: Ia046e2b57ebc56eaf0c3dc330d9f8e81763df8bc
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/20144
Reviewed-by: David Neto <dneto@google.com>

src/writer/spirv/builder.cc

@@ -201,6 +201,9 @@ uint32_t Builder::GenerateExpression(ast::Expression* expr) {
   if (expr->IsBinary()) {
     return GenerateBinaryExpression(expr->AsBinary());
   }
+  if (expr->IsCall()) {
+    return GenerateCallExpression(expr->AsCall());
+  }
   if (expr->IsConstructor()) {
     return GenerateConstructorExpression(expr->AsConstructor(), false);
   }