includePath support for non-local paths `ssh://` in particular

Type: LanguageService
----- Input information below -----

Please review existing issues and our documentation at https://github.com/Microsoft/vscode-cpptools/tree/master/Documentation prior to filing an issue.

Describe the bug

• OS and Version: Windows 7 Enterprise
• VS Code Version: 1.25.1
• C/C++ Extension Version: 0.17.6
• Other extensions you installed (and if the issue persists after disabling them): SSH FS 1.9.2

IntelliSense fails to parse files provided by extensions that implement FileSystemProvider. In my case: SSH-FS https://github.com/SchoofsKelvin/vscode-sshfs/issues/27

To Reproduce

1. install vscode-cpptools
2. install vscode-sshfs
3. configure a remote directory in user settings
4. configure c_cpp_properties.js includePath to use the ssh FileSystemProvider

Expected behavior
IntelliSense should work as it does for local files

User Settings:

"sshfs.configs": [
        {
            "name": "home",
            "label": "jwieber",
            "root": "/home/jwieber",
            "host": "my.host.domain",
            "port": 1234,
            "username": "jwieber",
            "agent": "C:\\cygwin64\\bin\\ssh-agent.exe",
            "privateKeyPath": "C:\\Users\\jwieber\\.ssh\\id_rsa.ppk"
        },
        {
            "name": "repo",
            "root": "/path/to/my/repo",
            "host": "my.host.domain",
            "port": 1234,
            "username": "jwieber",
            "agent": "C:\\cygwin64\\bin\\ssh-agent.exe",
            "privateKeyPath": "C:\\Users\\jwieber\\.ssh\\id_rsa.ppk"
        }
    ]

code-workspace

{
	"folders": [		
		{
			"path": "./fdmdbsvc"
		},
		{
			"uri": "ssh://home/",
			"name": "SSH FS - home"
		},
		{
			"uri": "ssh://repo/",
			"name": "SSH FS - repo"
		}
	],
	"settings": {
		"files.associations": {
			"*.ipp": "cpp"
		}
	}
}

c_cpp_properties.json

{
    "configurations": [
        {
            "name": "Win32",
            "includePath": [
                "${workspaceFolder}",
                "ssh://repo/"
            ],
            "defines": [
                "_DEBUG",
                "UNICODE",
                "_UNICODE"
            ],
            "compilerPath": "C:\\cygwin64\\bin\\gcc.exe",
            "intelliSenseMode": "clang-x64"
             "cStandard": "c11",
            "cppStandard": "c++17"
        }
    ],
    "version": 4
}

Reproduce the failure with the SSH FS workspace in code-workspace and the ssh://repo/ entry in c_cpp_properties.json. Start by tracing how includePath is consumed for IntelliSense and how FileSystemProvider-backed paths are handled; done means IntelliSense parses files from the remote path as it does local files.