[lldb-server] Improve support on Windows

Summary:
This commit contains the following changes:

  - Rewrite vfile close/read/write packet handlers with portable routines from lldb.
    This removes #if(s) and allows the handlers to work on Windows.

  - Fix a bug in File::Write. This is intended to write data at an offset to a file
    but actually writes at the current position of the file.

  - Add a default boolean argument 'should_close_fd' to FileSystem::Open to
    let the user decide whether to close the fd or not.

Reviewers: zturner, llvm-commits, labath

Reviewed By: zturner

Subscribers: Hui, labath, abidh, lldb-commits

Differential Revision: https://reviews.llvm.org/D56231

llvm-svn: 353446