[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>
diff --git a/src/writer/spirv/builder.cc b/src/writer/spirv/builder.cc
index 1928777..6f8032a 100644
--- a/src/writer/spirv/builder.cc
+++ b/src/writer/spirv/builder.cc
@@ -201,6 +201,9 @@
   if (expr->IsBinary()) {
     return GenerateBinaryExpression(expr->AsBinary());
   }
+  if (expr->IsCall()) {
+    return GenerateCallExpression(expr->AsCall());
+  }
   if (expr->IsConstructor()) {
     return GenerateConstructorExpression(expr->AsConstructor(), false);
   }