Make module loaders owned by the AST context instead of ref-counted.

Also, create the Clang module loader directly rather than indirecting through
a "get constructor" function. It's no longer a valid configuration to not
have a Clang importer.

Swift SVN r16862

include/swift/AST/ModuleLoader.h

@@ -30,7 +30,7 @@ class NominalTypeDecl;
 enum class KnownProtocolKind : uint8_t;
 
 /// \brief Abstract interface that loads named modules into the AST.
-class ModuleLoader : public llvm::RefCountedBaseVPTR {
+class ModuleLoader {
   virtual void anchor();
 
 public: