Add an optional language feature for Library Evolution

Replace the one-off compiler flag for Library Evolution with an
optional language feature. This makes the
`hasFeature(LibraryEvolution)` check work in an `#if`, and is
otherwise just cleanup.

Tracked by rdar://161125572.

include/swift/Basic/Features.def

@@ -308,6 +308,8 @@ UPCOMING_FEATURE(ImmutableWeakCaptures, 481, 7)
 /// safety.
 MIGRATABLE_OPTIONAL_LANGUAGE_FEATURE(StrictMemorySafety, 458, "Strict memory safety")
 
+OPTIONAL_LANGUAGE_FEATURE(LibraryEvolution, 0, "Library evolution")
+
 // Experimental features
 
 EXPERIMENTAL_FEATURE(StaticAssert, false)

include/swift/Frontend/FrontendOptions.h

@@ -298,11 +298,6 @@ public:
   /// \see ModuleDecl::isImplicitDynamicEnabled
   bool EnableImplicitDynamic = false;
 
-  /// Enables the "fully resilient" resilience strategy.
-  ///
-  /// \see ResilienceStrategy::Resilient
-  bool EnableLibraryEvolution = false;
-
   /// If set, this module is part of a mixed Objective-C/Swift framework, and
   /// the Objective-C half should implicitly be visible to the Swift sources.
   bool ImportUnderlyingModule = false;

include/swift/Sema/SourceLoader.h

@@ -25,20 +25,16 @@ class SourceLoader : public ModuleLoader {
 private:
   ASTContext &Ctx;
   std::vector<ModuleDecl *> ModulesToBindExtensions;
-  bool EnableLibraryEvolution;
 
   explicit SourceLoader(ASTContext &ctx,
-                        bool enableResilience,
                         DependencyTracker *tracker)
-    : ModuleLoader(tracker), Ctx(ctx),
-      EnableLibraryEvolution(enableResilience) {}
+    : ModuleLoader(tracker), Ctx(ctx) {}
 
 public:
   static std::unique_ptr<SourceLoader>
-  create(ASTContext &ctx, bool enableResilience,
-         DependencyTracker *tracker = nullptr) {
+  create(ASTContext &ctx, DependencyTracker *tracker = nullptr) {
     return std::unique_ptr<SourceLoader>{
-      new SourceLoader(ctx, enableResilience, tracker)
+      new SourceLoader(ctx, tracker)
     };
   }