Fix issue with cwd being set to untitled file path

This change fixes an issue with untitled file paths being sent in the
cwd parameter to debug adapter when in a window that doesn't have a
workspace loaded.  The fix is to change our cwd selection logic to be
more aware of an untitled file being used.

Resolves PowerShell#655.

Author: daviwil

src/features/DebugSession.ts

@@ -42,7 +42,10 @@ export class DebugSessionFeature implements IFeature {
 
         if (config.request === 'launch') {
             // Make sure there's a usable working directory if possible
-            config.cwd = config.cwd || vscode.workspace.rootPath || config.script;
+            config.cwd =
+                config.cwd ||
+                vscode.workspace.rootPath ||
+                (!currentDocument.isUntitled ? config.script : undefined);
 
             // For launch of "current script", don't start the debugger if the current file
             // is not a file that can be debugged by PowerShell
@@ -51,6 +54,7 @@ export class DebugSessionFeature implements IFeature {
                 if (currentDocument.isUntitled) {
                     if (currentDocument.languageId === 'powershell') {
                         config.script = currentDocument.uri.toString();
+                        config.cwd = vscode.workspace.rootPath;
                     }
                     else {
                         let msg = "To debug '" + currentDocument.fileName +