Frontend options should only be printed under `swift-module-flags-ignored:`
temporarily to prevent condfails for older development compilers that are still
in use (as a rule of thumb, swiftinterfaces should be compatible with compilers
that are up to 6 months old). None of the frontend flags that are currently
categorized as "ignorable" need to be ignorable anymore.
This new flag should allow module authors to specify allowable client list. This list is printed
in the textual module interface. Diagnostics support can come later.