Issue Type: Bug

The extension C/C++ cannot be run after upgrading to version 0.21.0-insiders2.

The last version (0.21.0) runs well.

Way to reproduce:

1. Upgrade the extension C/C++ to version 0.21.0-insiders2 (which may be performed automatically by VSCode - Insiders);
2. Reload the window when VSCode informs you of that the extension has been upgraded and requires you to do so.
3. VSCode will pop up two messages, which read:

Unable to start the C/C++ language server. IntelliSense features will be disabled. Error: Error: spawn UNKNOWN

and

Couldn't start client cpptools: Codes

while in the output window there appears the information of the error:

[Error - 下午4:51:09] Starting client failed
Error: spawn UNKNOWN
at ChildProcess.spawn (internal/child_process.js:357:11)
at Object.spawn (child_process.js:540:9)
at createMessageTransports._getServerWorkingDir.then.serverWorkingDir (.vscode-insiders\extensions\ms-vscode.cpptools-0.21.0-insiders2\node_modules\vscode-languageclient\lib\main.js:353:40)

Extension version: 0.21.0-insiders2
VS Code version: Code - Insiders 1.31.0-insider (e02e44464c02d617b98d0a3265bf5efef7794436, 2018-12-21T06:20:35.852Z)
OS version: Windows_NT x64 10.0.18305