Enable shadow warning by default

It's always a bad idea having multiple variables with the same name, so complain if that happens
7 jobs for add_shadow in 4 minutes and 19 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #374914
build

00:03:17

passed #374919
build_android

00:01:04

passed #374915
build_clang_libcpp

00:03:16

passed #374920
build_clazy

00:02:37

passed #374917
build_mingw64_fedora

00:02:14

passed #374916
build_ubuntu_16_04

00:01:40

failed #374918
build_clazy

00:00:49