[ClangImporter] Remove importer-based NS stripping. (#3880)

* [ClangImporter] Remove importer-based NS stripping.

As Tony puts it, in the end we wound up with more Foundation
declarations imported as members or keeping "NS" than those that
dropped it, and any further decisions will be made on a case-by-case
basis. Move all of the existing cases of prefix-stripping into
Foundation's API notes and drop the logic from the compiler.

Tested by dumping the generated interface for Foundation and its
submodules for both macOS and the iOS simulator, and comparing the
results. A few cases did slip through here because of the interaction
between "SwiftName" and "Availability: nonswift".

The next commit will re-add "NS" to some stragglers that we missed.

rdar://problem/26880017

* APINotes: Add "NS" back to a few types.

NSKeyedUnarchiverDelegate
NSKeyedArchiverDelegate
NSTextCheckingTypes
NSBinarySearchingOptions
NSEnumerationOptions
NSSortOptions

More rdar://problem/26880017

* Remove now-redundant SwiftNames from API notes.

No change observed in the generated interface of Foundation and its
submodules.

Finishes rdar://problem/26880017.

lib/Frontend/CompilerInvocation.cpp

@@ -771,7 +771,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.Playground |= Args.hasArg(OPT_playground);
   Opts.Swift3Migration |= Args.hasArg(OPT_swift3_migration);
   Opts.WarnOmitNeedlessWords = Args.hasArg(OPT_warn_omit_needless_words);
-  Opts.StripNSPrefix |= Args.hasArg(OPT_enable_strip_ns_prefix);
   Opts.InferImportAsMember |= Args.hasArg(OPT_enable_infer_import_as_member);
 
   Opts.EnableThrowWithoutTry |= Args.hasArg(OPT_enable_throw_without_try);