This moves language-mode initialization for MODULE_SCOPE from Scope::SetDefaults to the single constructor that needs to do it. The CL additionally makes it more obvious that scope-info-backed scopes always initialize language-mode unless WITH_SCOPE.

BUG=

Review-Url: https://codereview.chromium.org/2251683002
Cr-Commit-Position: refs/heads/master@{#38650}