Add @_neverEmitIntoClient to prohibit SIL serialization for a function

Part of the Embedded Swift linkage model, this attribute ensures that
the function it applies to has a strong definition in its owning
module, and that its SIL is never serialized. That way, other modules
will not have access to its definition.

Implements rdar://158364184.

include/swift/AST/Decl.h

@@ -1068,6 +1068,14 @@ public:
   /// behaviors for it and, if it's an extension, its members.
   bool isObjCImplementation() const;
 
+  /// True if this declaration should never have its implementation made
+  /// available to any client. This overrides cross-module optimization and
+  /// optimizations that might use the implementation, such that the only
+  /// implementation of this function is the one compiled into its owning
+  /// module. Practically speaking, this prohibits serialization of the SIL
+  /// for this definition.
+  bool isNeverEmittedIntoClient() const;
+
   using AuxiliaryDeclCallback = llvm::function_ref<void(Decl *)>;
 
   /// Iterate over the auxiliary declarations for this declaration,

include/swift/AST/DeclAttr.def

@@ -201,7 +201,11 @@ SIMPLE_DECL_ATTR(requires_stored_property_inits, RequiresStoredPropertyInits,
   ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | UnreachableInABIAttr,
   27)
 
-// Unused '28'
+SIMPLE_DECL_ATTR(_neverEmitIntoClient, NeverEmitIntoClient,
+  OnVar | OnSubscript | OnAbstractFunction,
+  UserInaccessible | ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIStableToRemove | ForbiddenInABIAttr,
+  28)
+
 // Unused '29'
 
 SIMPLE_DECL_ATTR(nonobjc, NonObjC,