Sink private-file import support down to SerializedASTFile

Implementing it in LoadedFile is nice in theory, but causes a leak in
practice because that type is ASTContext-allocated and usually never
destroyed.

https://bugs.swift.org/browse/SR-11366

lib/Serialization/SerializedModuleLoader.cpp

@@ -955,6 +955,11 @@ void SerializedASTFile::lookupValue(ModuleDecl::AccessPathTy accessPath,
   File.lookupValue(name, results);
 }
 
+StringRef
+SerializedASTFile::getFilenameForPrivateDecl(const ValueDecl *decl) const {
+  return File.FilenamesForPrivateValues.lookup(decl);
+}
+
 TypeDecl *SerializedASTFile::lookupLocalType(llvm::StringRef MangledName) const{
   return File.lookupLocalType(MangledName);
 }