C# extension should have an equivalent to VS's "No Source" window

## Environment data
`dotnet --info` output:

.NET Command Line Tools (2.0.0)

Product Information:
Version: 2.0.0
Commit SHA-1 hash: cdcd1928c9

Runtime Environment:
OS Name: Mac OS X
OS Version: 10.12
OS Platform: Darwin
RID: osx.10.12-x64
Base Path: /usr/local/share/dotnet/sdk/2.0.0/

Microsoft .NET Core Shared Framework Host

Version : 2.0.0
Build : e8b8861ac7faf042c87a5c2f9f2d04c98b69f28d

VS Code version: 1.17.1
C# Extension version: 1.12.1

## Steps to reproduce
Compile .NET Core app on Windows with portable debug info
- note: App is built in VS running in Parallels. Solution is on Mac drive mapped via Parallels to Windows.

Run it on macOS
Attach from VS Code
Debugger attaches but cannot find source and there seem to be no way to point debugger to the source location


## Expected behavior
Ability to open source files like in VS - Attach | Debugger asks where the source is.

## Actual behavior