Add backslach to make class explict in global namespace

Merge 3.5.0.1 to 3.5.x

* Fix ruby gc_test in ruby 2.4

* Initialize global variables to Qnil.

* Add discard unknown API in ruby.

* Add test for oneof message field.

* Add TestUnknown to represent unknown field data clearly.

* Only serialize the message with unknown fields itself in test.

* Move discard_unknown from Message to Google.Protobuf

* Recursively clear unknown fields in submessages.

* Recursively discard unknown fields in submsg for c extension

* Fix zts build

* Add comment for tests

* Add a TODO to add a util for encoding varint for better readability.

* Add test for oneof message field.

io_win32: support non-ASCII paths

Unlike GetEnvironmentVariableW,
GetCurrentDirectoryW doesn't set
ERROR_INSUFFICIENT_BUFFER.

If the test cannot find a temp directory by
checking environment variables, it will fall back
to using the current working directory as the temp
directory root.

This is what the test used to do as of commit
https://github.com/google/protobuf/commit/6de51caed52d798815954646b230c5aef3e4d2fc
and what was then changed by commit
https://github.com/google/protobuf/pull/3978/commits/792d098769d8e000d8d474c8ffd201d2eabc2134

Fixes https://github.com/google/protobuf/issues/3951

Do not use "googletest.h", apprently that leads to
linking errors on Windows which I couldn't figure
out how to solve, and decided to just go with
plain gTest instead.

See https://github.com/google/protobuf/issues/3951

* Provide discardUnknownFields API in php implementation

* Provide discardUnknownFields API in php c extension.

Add _file_desc_by_toplevel_extension back

Add _file_desc_by_toplevel_extension back for compatibility issue with 3.4.1.

_file_desc_by_toplevel_extension is a temporary solution for FieldDescriptor.file.
FieldDescriptor.file was added in code gen. However, we need to keep it for some time for compatibility reason (with 3.4.1 only). May try to remove it after 2 years.

Add protobuf-all in post release

conflict

Support win32 long path for cross compiled build

Update changelog