Convert FileSystem::Open() to return Expected<FileUP>

Summary:
This patch converts FileSystem::Open from this prototype:

Status
Open(File &File, const FileSpec &file_spec, ...);

to this one:

llvm::Expected<std::unique_ptr<File>>
Open(const FileSpec &file_spec, ...);

This is beneficial on its own, as llvm::Expected is a more modern
and recommended error type than Status.  It is also a necessary step
towards https://reviews.llvm.org/D67891, and further developments
for lldb_private::File.

Reviewers: JDevlieghere, jasonmolenda, labath

Reviewed By: labath

Subscribers: mgorny, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D67996

llvm-svn: 373003

lldb/source/Plugins/ScriptInterpreter/Python/PythonDataObjects.h

@@ -455,7 +455,6 @@ class PythonFile : public PythonObject {
 public:
   PythonFile();
   PythonFile(File &file, const char *mode);
-  PythonFile(const char *path, const char *mode);
   PythonFile(PyRefType type, PyObject *o);
 
   ~PythonFile() override;
@@ -469,7 +468,7 @@ public:
 
   static uint32_t GetOptionsFromMode(llvm::StringRef mode);
 
-  bool GetUnderlyingFile(File &file) const;
+  lldb::FileUP GetUnderlyingFile() const;
 };
 
 } // namespace lldb_private