swift234

Improve omit-needless-words API dumping script to dump SDKs.

When invoking omit-needless-words without a module argument (-m), the
script will now dump the API of all of the frameworks in the provided
SDKs. One can also specify multiple SDK arguments, e.g.,

  omit-needless-words.py -s iphoneos macosx

to dump the APIs for the modules of multiple SDKs. The invocation

  omit-needless-words.py -s macosx iphoneos watchos appletvos

will dump everything in one shot.

One can still dump individual modules with "-m", but you'll now have
to specify an SDK from which to retrieve the module.