implement support for vFile:mode and vFile:size packets

[andrurogerz]

Purpose

Implement responses to the vFile:mode and vFile:size packets in ds2 platform and debug server sessions. Fully addresses issue #136.

Overview

• Add File::fileMode support to the Host::File interface
• Implement the POSIX version of Host::File::fileMode using stat(2)
• Stub-out the Windows implementation of Host::File::fileMode
• Override and implement the existing SessionDelegate::onFileGetSize method in FileOperationsMixin to call the exiting Host::File::fileSize method
• Add a new SessionDelegate::onFileGetMode method and override it in FileOperationsMixin to call the new Host::File::fileMode method
• Fix error response of vFile:size to match the lldb documentation.

Problem Details

DS2 does not properly implement responses to the vFile:mode and vFile:size packets. These packets are documented here. Most of the pieces for vFile:size were already present, but they were not all tied together.

Validation

With some other uncommitted local patches, verify the following lldb test cases pass on Android and Linux:

TestGdbRemotePlatformFile.TestGdbRemotePlatformFile.test_platform_file_mode_fail_llgs
TestGdbRemotePlatformFile.TestGdbRemotePlatformFile.test_platform_file_mode_llgs
TestGdbRemotePlatformFile.TestGdbRemotePlatformFile.test_platform_file_size_llgs

These test cases will be re-enabled in CI testing after this and a number of other patches are merged.